WASHINGTON (Reuters) - Senate hearings began Tuesday on a treaty banning the testing of nuclear weapons, as President Clinton pressed on with his uphill case for ratification of the 152-nation agreement ahead of a snap vote.

CIA and U.S. Energy Department experts Tuesday testified behind closed doors to the Senate Armed Services Committee on the military implications of the nation Comprehensive Test Ban Treaty (CTBT), which the Senate's Republican leadership last week scheduled for a vote Oct. 14 after a two-year delay.

The armed services panel and the Senate Foreign Relations Committee, headed by Republican treaty foe Sen. Jesse Helms of North Carolina, Monday reversed earlier plans not to hold hearings and said they would take testimony this week.

The White House drew some encouragement from the decision to hold hearings, but remained guarded about the prospects of avoiding what would be a major foreign policy defeat if it were to lose the vote or be forced to pull the treaty.

"Even at the 11th hour it's important that the Senate deal with this in a responsible way and deal with the important issues that face us. They will deal with no more serious issue about our security and safety for Americans into the next century," White House spokesman Joe Lockhart said.

"Whatever the reasons for scheduling the hearing, it's important that the Senate hear the arguments from our scientific community, from our national security community and from a bipartisan group of former national security officials that it is in our interest," Lockhart said.

The Republican-controlled Senate, which has dragged its feet over the treaty for two years by tying its ratification to other matters, last week abruptly decided to begin debating it this Friday, with a vote set for next Tuesday.

Clinton, who began a blitz campaign for ratification Monday, has admitted that he lacked the 67 votes needed to ratify the treaty, which bans nuclear testing and provides for a worldwide network of sensors as well as on-site inspections to ensure that nations are not conducting tests.

White House officials said only two Senate Republicans were on record as favoring the treaty, but they were hoping other moderates could be won over. All 45 Democrats in the 100-member Senate are expected to vote for the treaty.

Analysts said a key voice could be Republican presidential candidate George W. Bush, who has not commented publicly on it.

Clinton was planning to hold a dinner for about six senators -- some treaty supporters and some undecided -- at the White House Tuesday evening.

He also was planning to speak for the treaty while signing a $288.8 billion defense programs authorization bill at the Pentagon Tuesday afternoon.

In addition, the White House planned to bring several security and foreign policy experts in the administration to brief reporters at the White House on the treaty.

Proponents say the treaty, which Clinton signed in 1996 and sent to the Senate the following year, would deter other nations from developing and refining nuclear weapons. Opponents argue that compliance with the treaty cannot be adequately verified.

In order to enter into force, the treaty must be ratified by all 44 countries in the Geneva-based Conference on Disarmament that have nuclear reactors or research programs, including the United States. Twenty-three have done so, according to the Vienna-based Comprehensive Nuclear Test Ban Treaty Organization.

Less than a third of the 152 countries that have signed the treaty have ratified it. U.S. officials said that Washington's stance was key to persuading others to follow suit.