The Senate Thursday began its formal debate on John Tower's troubled nomination as defense secretary, and two additional Democrats joined the ranks of the opposition, but a key Republican insisted that confirmation is still "winnable."

At the White House, spokesman Marlin Fitzwater conceded that despite two days of personal lobbying, President Bush had not yet been able to persuade any Democrats to line up in favor of confirmation.Sen. Sam Nunn, who chairs the Armed Services Committee, led off the floor debate and said he was acting with a "sense of sorrow" in leading the opposition to his former colleague.

Nunn, D-Ga., said that when the committee began its hearings into Tower's nomination he expected that he would in "good faith support the nominee. My mind was changed by the facts presented to the committee."

With Republicans trying to spend time marshaling public support on behalf of the nomination, no vote is expected until next week.

Tower has been battling questions of drinking habits, allegations of womanizing and questions over possible conflicts of interest stemming from his work as a defense industry consultant.

The debate began as aides to Sens. Alan Cranston of California and Terry Sanford of North Carolina said both men would vote against the nomination - bringing the number of Democrats in opposition to 38.

Appearing on ABC's "Good Morning America," Sen. John Warner, R-Va., said, "I think it's winnable in the sense that we get the American public involved in this. And as soon as this debate starts I'm confident that they'll communicate with the senators. It's important that we not have a soap opera or a civil war . . . but a fair and factual debate," said Warner, the ranking Republican on the Armed Services Committee.

Cranston, the Senate Democratic whip, said through an aide that he would vote against the nomination. An aide to Sanford, Rafe Greenlee, said his boss had come to the same conclusion after "closely following the hearings and reading the FBI report" on Tower's background check.

An Associated Press tally at midday showed 43 Democrats and Republican Larry Pressler of South Dakota either solidly opposed or leaning in opposition, and 40 Republicans either solidly or leaning in favor. Democrats hold a 55-45 majority in the Senate.

Fitzwater said President Bush still believes all allegations against Tower are unfounded. He praised the nominee's combative appearance on Wednesday at a question-and-answer session at the National Press Club.

"We thought Senator Tower's performance yesterday was excellent. It showed that he's a fighter and a scrappy guy and just the man to take over the Defense Department," the spokesman said.

Fitzwater said the White House is in full accord with Senate Republican Leader Bob Dole's plans to delay a final vote. "We think everybody should have a chance to register their vote. And you never know, you might change," Fitzwater said. But, when asked if the White House could name one Democrat who had been won over, he said: "Not yet."

Tower, meanwhile, attended a morning meeting at the White House with Bush, Adm. William Crowe, chairman of the Joint Chiefs of Staff; and acting Defense Secretary William H. Taft IV.

Fitzwater said the meeting dealt with Pentagon management reforms and an ongoing review of strategic issues, and not specifically with Tower's confirmation battle.

Tower said Wednesday he has no intention of giving up. "Never surrender or retreat," the former Texas senator said at his press club appearance, quoting the commander of the Texas forces at the Alamo.

Tower renewed his vow to abstain from drinking if he is confirmed. Asked if he had ever broken any previous pledges, including wedding vows, the twice-divorced Tower said he had broken marital vows. "I think I probably am not alone in that connection," he added.

Tower also said he would not return to the defense consulting industry even if his confirmation fails.

Bush continued to lobby individual senators on the Tower nomination. He summarized his appeal to them this way: "Do what you've got to do, but remember fair play, remember decency and honor and then remember also historically the concept of advice and consent where reasonable doubt is given historically to the president who after all is responsible for the executive branch of this government."

Sen. William Armstrong, R-Colo., complained Wednesday that information in a confidential FBI report about Tower should be made public. White House counsel C. Boyden Gray traveled to the Capitol to meet with key senators to discuss whether any of the information could be made public.

Some of the specific allegations against Tower have surfaced in news reports. The Washington Post Thursday quoted a retired Air Force sergeant as saying Tower appeared drunk and fondled women on two visits to Bergstrom Air Force Base in Texas, in 1976 and 1978.

Fitzwater, the White House spokesman, dismissed those allegations as "more innuendo and rumor."