WASHINGTON--John Tower, his nomination as defense secretary in grave trouble, made an extraordinary public admission of marital infidelity yesterday but vowed to keep fighting for Senate confirmation. His prospects dimmed further, however, by the Senate majority leader's opposition.

President Bush continued to court Democrats in a last-ditch effort to save the appointment, and spokesman Marlin Fitzwater asserted, "We'll just keep up the good fight."

But Senate Majority Leader George Mitchell further dampened Tower's chances by declaring his opposition, even as Vice President Dan Quayle conceded the administration still lacks the votes needed for Senate approval.

The president said his appeal to senators has been, "Look, do what you've got to do, but remember fair play, remember decency and honor and then remember also historically the concept of advise and consent where reasonable doubt is given historically to the president of the United States who after all is responsible for the executive branch of this government."

Bush dispatched Quayle to Capitol Hill to lobby for the nomination.

"We hope the prospects improve. We don't have 50 votes yet. But we're moving in that direction," Quayle told a White House photo session.

Tower showed flashes of grit and humor as he fielded questions at the National Press Club about allegations concerning excessive drinking, womanizing and lucrative consulting fees he received from defense contractors.

Asked whether he would ask that his name be withdrawn, he replied by quoting from the last letter written by Col. William Barrett Travis, the leader of the Texans who fought and died at the Alamo: "I shall never surrender or retreat."

The former Texas senator paused briefly and said, "I am a little sorry I brought up the Alamo analogy. It just occurred to me what happened at the Alamo just 10 days after Travis wrote that letter." Texas defenders were wiped out by the Mexicans.

Tower said he regards his weekend vow to quit drinking if he wins confirmation as "sacred...and I can assure you I will abide by it."

Asked whether he had ever broken his wedding vows, the twice-divorced Tower said, "As a matter of fact, I have broken wedding vows. I think I am probably not alone in that connection."

Tower's second wife, Lilla Burt Cummings, alleged "marital misconduct" in court papers in the couple's divorce case.

There was no letup, meanwhile, in the political storm surrounding the nomination.

Republicans were trying to decide whether to seek a delay in the beginning of the debate on the nomination, which has drawn widespread opposition from Democrats.

The administration is trying to hold all 45 Republicans and sway five of the Senate's 55 Democrats to get a tie vote that Quayle could break in favor of Tower.

An Associated Press survey of the Senate found 36 Republicans supporting the nomination, 32 Democrats opposed and 29 undecided or undeclared. Two senators were unavailable for comment.

Mitchell (D-Maine), the Democratic leader, announced his opposition to the nomination in a midday speech on the Senate floor, citing Tower's ties to defense consultants and his drinking.

Mitchell said he read the FBI report on Tower and met with Bush at the White House where he expressed his concerns about the nomination.

"It is only after careful study of this record that I have concluded that John Tower is not the man to provide leadership by example to the military and civilian employees of the Department of Defense, especially in the difficult job of reforming the Pentagon's administration and procurement practices in a period of budget stringency," Mitchell said.

Tower lashed out at what he said was the absence of clearly defined standards against which he was being judged. He noted that several senators have said that the secretary of defense, as the commander of U.S. military forces behind only the president, must adhere to a higher standard. "I accept that...but my question is how much," he said.