President Bush plunged into an uphill campaign to salvage his nomination of John Tower as defense secretary with a series of private meetings Tuesday intended to sway senators to vote for confirmation.

But the nomination suffered another setback when Republican Larry Pressler of South Dakota announced he was leaning against confirmation and urged Tower to withdraw."I think that he may very well be doing him (Bush) a favor," said Pressler, who said he was fearful that Tower would not be capable of cleaning up the Pentagon procurement system if confirmed.

Pressler's comments were the most public indication to date that Republican support for Tower - and the president - might not be firm.

With Democrats holding a 55-45 majority in the Senate, Republicans can ill-afford any defections from their own ranks if Bush is to prevail in the first high-stakes political showdown of his 5-week-old presidency.

Vice President Dan Quayle entered the fray, telling reporters at a hastily called news conference at the entrance to the Senate subway that the White House is in an all-out drive to confirm Tower.

Quayle said he believes Tower himself already has removed a serious obstacle through his pledge of abstinence and that when the vote is taken all 45 Republican senators, including Pressler, will vote for Tower.

"We are still searching for five Democrats, and I'll make the 51st vote in favor of Tower," said Quayle, who as Senate president is empowered to cast tie-breaking votes.

Bush invited several Democrats to the White House for a series of one-on-one meetings Tuesday. The first, Majority Leader George Mitchell of Maine, did not talk with reporters afterward but said he would announce his own position by day's end.

With the full Senate prepared to take up the nomination this week, Bush rejoined the battle Monday night after returning to the capital from the Far East. He told reporters on Air Force One: "I haven't wavered one iota and I don't intend to."

Democrats have been largely unimpressed with Tower's pledge to swear off alcohol, saying they are concerned with the issue of his judgment in general and his past links with military contractors. Bush wants to change their minds.

In addition to Mitchell, Bush met with John Warner of Virginia, the ranking Republican on the Senate Armed Services Committee, and five Democrats whose votes are considered crucial - Sens. Charles Robb, D-Va.; Lloyd Bentsen, D-Texas; Bennett Johnston, D-La., Dennis DeConcini, D-Ariz., and Bill Bradley, D-N.J.

"We're going to work hard," said White House press secretary Marlin Fitzwater. "We're going to talk to a lot of people and ask them to keep an open mind and to read the FBI report and to talk to John Tower."

Following a White House meeting Monday night, Senate Republican leader Bob Dole, R-Kan., said there was no talk of pulling the plug on Tower's nomination.

Senators are using the time before formal debate opens Wednesday to troop into a guarded Capitol hearing room to read a confidential FBI report on the former Texas senator's drinking habits, alleged "womanizing" and the extensive and lucrative links he forged to military contractors after retiring from the Senate four years ago.

Pressler, interviewed on "CBS This Morning," Tuesday said, "I've said I'm leaning very much against him and probably will vote against him. It's based on a strong feeling that John Tower is very much a part of the problem. He comes out of the very group that, that in my view has made the contractor problem over there."

However, Sen. Arlen Specter, R-Pa., told CBS he leaned toward supporting Tower, adding: "It is not a Supreme Court appointment. Tower would serve at the pleasure of the president. He could fire him at any time. If John Tower were to drink, become inebriated, under current circumstances it would be known everywhere in the world virtually instantaneously."