WASHINGTON--The Senate confirmation battle over Defense Secretary-designate John Tower neared a climax yesterday as Majority Leader George Mitchell proclaimed the nomination doomed and Republican leader Bob Dole conceded it would "take a miracle" to stave off defeat.

Lloyd Bentsen of Texas became the third Democrat to endorse Tower's confirmation, but his speech was followed by a declaration of opposition from Bennett Johnston of Louisiana, who had been lobbied hard by the Bush administration as a potential pro-Tower vote.

Wavering Democrats Charles Robb of Virginia and Joseph Lieberman of Connecticut spread the word that they would announce their intentions before the day was out.

Republicans refused to concede defeat and Dole said President Bush would "buy lunch" for any Democrat willing to listen.

But later, after Johnston announced his intentions, Dole admitted that "it would take a miracle" for Tower to win confirmation.

"In the nuclear age we can't afford to take a chance on the man who holds the future in his hands," Johnston said on the Senate floor.

Johnston said he was troubled by Tower's drinking habits and the fact that alcohol altered the former senator's judgement.

"There seems to be a personality change that occurs in Senator Tower," said Johnston, who said he had received two phone calls about recent incidents involving Tower.

"I don't mean to use the term alcoholic," Johnston said. "I know he's an abuser."

Earlier, Bentsen became the third Democrat to support the embattled nomination, saying, "I hope that when I get the next chance to talk with John Tower about the tough job of keeping this country strong, it will be while he's in his new office at the Pentagon."

In a statement, Tower called his fellow Texan "a true statesman."

"I'm both honored to have his prestigious support, and grateful for his humane understanding of what my family has endured," Tower said.

Despite Bentsen's pledge of support, Mitchell repeated his claim that the full Senate will reject the nomination, and he led a chorus of Democrats who called for a quick vote to end the rancorous debate that dragged on for the fifth day.

Asked earlier in the day if Bentsen's decision could change the final outcome, Mitchell replied: "Whatever his decision I still have the ample votes."

But Senate Minority Leader Bob Dole of Kansas said "two more votes will give us a secretary of defense, maybe this week."

An Associated Press survey found 48 Democrats and Republican Sen. Larry Pressler of South Dakota either solidly opposed or leaning against confirmation with 41 Republicans and three Democrats either supporting or leaning in favor of the nomination.

Mitchell said he was ready to vote "as soon as possible. A reasonable time would be tomorrow."

Dole acknowledged that the debate has extended beyond the usual time frame for most nominees but said "this is unprecedented. We don't want to prolong the debate--we want to win."

Sens. Alan Cranston (D-Calif.) and Claiborne Pell (D-R.I.) announced that they would oppose the Tower nomination, with Pell's decision a disappointment to the White House, which had worked to win over the chairman of the Senate Foreign Relations Committee.

Pell cited Tower's lucrative ties to defense contractors, saying it would be difficult for Tower "to perform as secretary without certainly at least the appearance of conflict of interest."