The previously undisclosed effort has almost doubled in projected cost -- from $5 billion to nearly $9.5 billion, officials said. The National Reconnaissance Office, which manages spy satellite programs, has already spent hundreds of millions of dollars on the program, officials said.

The stealth satellite, which would probably become the largest single-item expenditure in the $40 billion intelligence budget, is to be launched in the next five years and is meant to replace an existing stealth satellite, according to officials. Non-stealth satellites can be tracked and their orbits can be predicted, allowing countries to attempt to hide weapons or troop movements on the ground when they are overhead.

Opponents of the new program, however, argue that the satellite is no longer a good match against today's adversaries: terrorists seeking small quantities of illicit weapons, or countries such as North Korea and Iran, which are believed to have placed their nuclear weapons programs underground and inside buildings specifically to avoid detection from spy satellites and aircraft.

The National Reconnaissance Office and the CIA declined to comment. Lockheed Martin Corp., which sources said is the lead contractor on the project, issued a statement saying, "As a matter of policy we do not discuss what we may or may not be doing in regards to classified programs."

The satellite in question would be the third and final version in a series of spacecraft funded under a classified program once known as Misty, officials said.

Concerned about the latest satellite's relevancy and escalating costs, the Senate Select Committee on Intelligence has twice tried to kill it, according to knowledgeable officials. The program has been strongly supported, however, by Senate and House appropriations committees; by the House intelligence committee, which was chaired by Rep. Porter J. Goss (R-Fla.) until he recently became CIA director; and by his predecessor, George J. Tenet.

"With the amount of money we're talking about here, you could build a whole new CIA," said one official, who, like others, talked about the program and the debate on the condition of anonymity because of the project's sensitivity.

The debate over the secret program has been carried out in closed session on Capitol Hill, and no legislator has publicly acknowledged the existence of the program. Echoes of the heated discussion, however, have begun to emerge in public.

Earlier this week, four Democratic senators refused to sign the "conference sheets" used by the House-Senate conference committee working on the 2005 intelligence authorization bill. Sources said that was meant to protest inclusion once again of the satellite program.

A statement by conference managers said only that four Democratic senators -- John D. Rockefeller IV (W.Va.), vice chairman of the intelligence committee; Carl M. Levin (Mich.); Richard J. Durbin (Ill.); and Ron Wyden (Ore.) -- objected to a classified item in the bill "that they believe is unnecessary and the cost of which they believe is unjustified." It continued: "They believe that the funds for this item should be expended on other intelligence programs that will make a surer and greater contribution to national security." Some Republican lawmakers have concerns about the program as well, as do some senators on the Armed Services Committee, sources said.

In an attempt to verbalize frustration while abiding by classification constraints, Rockefeller made an unusual reference to his protest on the Senate floor.

"My decision to take this somewhat unprecedented action is based solely on my strenuous objection -- shared by many in our committee -- to a particular major funding acquisition program that I believe is totally unjustified and very wasteful and dangerous to national security," Rockefeller said. "Because of the highly classified nature of the programs contained in the national intelligence budget, I cannot talk about them on the floor."

Rockefeller added that the committee has voted "to terminate the program" for the past two years, "only to be overruled" by the appropriations committees.

A small firestorm followed, with at least one radio talk show host and callers to Rockefeller's office charging that he had divulged classified information. On Thursday, spokeswoman Wendi Morigi issued what she called a clarification. "Any assertion about classified intelligence programs based on Senator Rockefeller's statement is wholly speculative," the statement said. It said Rockefeller's floor statement had been "fully vetted and approved by security officials."

That statement illustrates the constraints faced by members of Congress as they work to adjust or terminate even multibillion-dollar programs that are hidden from public scrutiny and debate. There have been other hints of problems in satellite programs in the last year.

Several months ago, Sen. Dianne Feinstein (D-Calif.), a member of the intelligence committee, made a cryptic reference to the value of expensive satellite programs during testimony on her intelligence reform proposal.

"I can't go into this, but when we look at satellites, one or the other of us has questions," she told her colleagues. "I'm concerned these are tens-of-billions-of-dollar items and we sure as heck better know what we're doing."

Stealth technology has been used to cloak military aircraft such as the F-117A fighter and the B-2 bomber.

When radar searches for a stealth craft, it records a signature that is much smaller than its size should indicate. Thus a stealth plane or satellite could appear to radar analysts as airborne debris.

Advanced nations routinely patrol the skies with radar and other equipment to detect spy planes, satellites and other sensors.

About 95 percent of spycraft are detected by other nations, experts say. But "even France and Russia would have a hard time figuring out what they were tracking" if they were to pick up the image of a stealth satellite, said John Pike of GlobalSecurity.org, an expert on space imagery.

The idea behind a stealth satellite is "so the evildoers wouldn't know we are looking at them," Pike said. "It's just a fundamental principle of operational security that you know when the other guy's satellites are going to be overhead and you plan accordingly."

But, Pike said, "the cover and deception going on today is more systematic and continual. It's not the 'duck and cover' of the Soviet era."

The existence of the maiden stealth satellite launched under the Misty program was first reported by Jeffrey T. Richelson in his 2001 book "The Wizards of Langley: Inside the CIA's Directorate of Science and Technology." Richelson said that first craft was launched from the space shuttle Atlantis on March 1, 1990.

Amateur space trackers in England and Canada were able to detect it at points after that, Richelson reported.

A second Misty satellite was launched nearly a decade later and is in operation, sources said.

Circumstantial evidence of that satellite's existence was outlined in the April issue of a Russian space magazine, Novosti Kosmonavtiki. According to a translation for The Washington Post, the article suggested that a satellite launched from Vandenberg Air Force Base in California in 1999 may be the second-generation Misty craft and noted that the satellite was put into orbit along with "a large number of debris," a likely deception method.

Researcher Julie Tate contributed to this report.

For two years, the senators have disclosed, Republicans and Democrats on the panel have voted to block the secret program, which is believed to be a system of new spy satellites. But it continues to be financed at a cost that former Congressional officials put at hundreds of millions of dollars a year with support from the House, the Bush administration and Congressional appropriations committees.

Senator John D. Rockefeller IV of West Virginia, the ranking Democrat on the panel, denounced the program on Wednesday on the Senate floor as "totally unjustified and very, very wasteful."

Senator Ron Wyden, Democrat of Oregon, later called it "unnecessary, ineffective, over budget and too expensive."

Neither senator would say much more about what he was referring to. Even in private on Thursday, most Congressional and intelligence officials who were asked refused to comment about the name, purpose or cost of the program. But former Congressional and intelligence officials who oppose it said it would duplicate capabilities in existence or in development, as part of the country's vast network of satellites, aircraft and drones designed for eavesdropping and reconnaissance.

Among the possibilities suggested by private experts, including John Pike of Globalsecurity.org, a research organization in Alexandria, Va., were that the system might be a controversial unproven program to launch a reconnaissance satellite that adversaries could not detect. Former Congressional officials said they would discount speculation that the debate had to do with any antisatellite space warfare capability.

A number of satellite programs in development, including a Future Imaging Architecture system that Boeing is developing, have been the subject of considerable public controversy, because of technical
problems and cost overruns. But current and former government officials said they did not believe that the Boeing program was the subject of the new dispute.

In addition to Mr. Rockefeller and Mr. Wyden, two other Democratic senators made their opposition public on Wednesday, saying the money dedicated to the acquisition program could better be transferred to
other intelligence gathering as part of what is widely understood to be the $40 billion intelligence budget.

The program being disputed by the senators is to be financed this year, but current and former government officials said Republicans as well as Democrats intended to redouble their efforts to block it.

The White House and the Central Intelligence Agency did not respond to a request for comment about the dispute. The Republican chairman of the House military appropriations subcommittee, whose support for the program has been instrumental in keeping it alive, also did not respond to a request for comment.

The most specific public hints on the program were by Mr. Wyden, who said on the Senate floor, "This issue must be highlighted, because it is not going away."

"Numerous independent reviews," he said, "have concluded that the program does not fulfill a major intelligence gap or shortfall, and the original justification for developing this technology has eroded in importance due to the changed practices and capabilities of our adversaries. There are a number of other programs in existence and in development whose capabilities can match those envisioned for this program at far less cost and technological risk."

The Senate Intelligence Committee first expressed concern about the program three years ago, and it has voted to block it for the last two years, Congressional officials said. A former Defense Department official said of the program: "This is something that does not pass muster and is indicative of the inability of intelligence agencies to prioritize or make decisions. There are billions of dollars of waste in the intelligence budget."

A former Congressional official said that "hard decisions should have been made to make choices" when Congress first authorized and appropriated the money several years ago. "Instead," the former official said, "the decision was made to just go ahead with go with everything."

Even the $40 billion figure attached to the current intelligence budget remains no more than an estimate, because spending figures remain classified by law. But much of the budget is widely understood to be devoted to the design, construction and operation of satellites and other platforms used to collect images, signals and other forms of technical intelligence.

Many critics have long complained that human intelligence programs remain underfinanced, at least in relative terms. In a directive last month, President Bush asked the C.I.A. to spell out a plan and a timetable to increase its clandestine service by 50 percent.

A compromise negotiated between the House and Senate this week provides authorization for continued financing for the disputed program. It was approved by 13 of the 17 senators on the Intelligence Committee and all of their House counterparts.

Because the financing had been approved in a military appropriations bill, Congressional officials said, the authorizing committees did not have the power to transfer the money to other intelligence programs.

But an unclassified version of the conference report released on Wednesday reported that Senators Carl Levin of Michigan and Richard J. Durbin of Illinois, both Democrats, along with Mr. Rockefeller and Mr. Wyden, had refused to sign the compromise.

The report said the senators believed that the money dedicated for what was described only as "a major acquisition program" ought to be "expended on other intelligence programs that will make a surer and greater contribution to national security."

In an unusually public rebuke of a secret government project, Sen. Jay Rockefeller of West Virginia, the senior Democrat on the Senate Intelligence Committee, complained Wednesday that the program was "totally unjustified and very, very wasteful and dangerous to the national security." He called the program "stunningly expensive."

Rockefeller and three other Democratic senators - Richard Durbin of Illinois, Carl Levin of Michigan and Ron Wyden of Oregon - refused to sign the congressional compromise negotiated by others in the House and Senate that provides for future U.S. intelligence activities.

The compromise noted that the four senators believed the mystery program was unnecessary and its cost unjustified and that "they believe that the funds for this item should be expended on other intelligence programs that will make a surer and greater contribution to national security."

Each senator - and more than two dozen current and former U.S. officials contacted by The Associated Press - declined to further describe or identify the disputed program, citing its classified nature. Thirteen other senators on the Intelligence Committee and all their counterparts in the House approved the compromise.

Despite objections from some in the Senate, Congress has approved the program for the past two years, Rockefeller said.

The Senate voted to send the legislation to President Bush on Wednesday night. The bill is separate from the intelligence overhaul legislation that also won final congressional approval Wednesday.

The rare criticisms of a highly secretive project in such a public forum intrigued outside intelligence experts, who said the program was almost certainly a spy satellite system, perhaps with technology to destroy potential attackers. They cited tantalizing hints in Rockefeller's remarks, such as the program's enormous expense and its alleged danger to national security.

A U.S. panel in 2001 described American defense and spy satellites as frighteningly vulnerable, saying technology to launch attacks in space was widely available. The study, by a commission whose members included Donald H. Rumsfeld prior to his appointment as defense secretary for Bush, concluded that the United States was "an attractive candidate for a Space Pearl Harbor."

Sending even defensive satellite weapons into orbit could start an arms race in space, warned John Pike, a defense analyst with GlobalSecurity.org, who has studied anti-satellite weapons for more than three decades. Pike said other countries would inevitably demand proof that any weapons were only defensive.

"It would present just absolutely insurmountable verification problems because we are not going to let anybody look at our spy satellites," Pike said.

Rockefeller's description of the spy project as a "major funding acquisition program" suggests a price tag in the range of billions of dollars, intelligence experts said. But even expensive imagery or eavesdropping satellites, as long as they're unarmed, are rarely criticized as a danger to U.S. security, they noted.

"From the price, it's almost certainly a satellite program," said James Bamford, author of two books about the National Security Agency.

Another expert agreed. "It's hard to think of most any satellite program, at least the standard ones, as dangerous to national security," said Jeffrey T. Richelson, who wrote a highly regarded book about CIA technology in 2001.