WASHINGTON - Sen. Bill Frist of Tennessee is poised to challenge Sen. Trent Lott of Mississippi for Lott's post of Senate Republican leader, he announced last night after a handful of influential senators threw their support behind him.

Frist's announcement dealt a serious blow to Lott's intensive campaign to keep his post as majority leader. It came as Lott waged a behind-the-scenes effort to garner the 26 votes he would need. After Frist's statement last night, Lott vowed to continue fighting for his leadership job.

Having spent days declining to comment on a potential leadership battle, Frist - a close ally of President Bush and a rising Republican star - said he had decided to challenge Lott if enough of the 50 other Republican senators asked him to.

After speaking with supporters, Frist said, "I indicated to them that if it is clear that a majority of the Republican caucus believes a change in leadership would benefit the institution of the United States Senate, I will likely step forward for that role."

In recent days, Frist, a heart surgeon who was a key architect of his party's midterm election gains, had cultivated the image of one reluctant to challenge the party's Senate leader.

Within hours of Frist's statement, one of the most influential senior senators, John W. Warner of Virginia, removed all hint of uncertainty about Frist's intentions as he came out in strong support of him.

"There's a fast-moving momentum building up for Bill Frist," declared Warner, who is set to become chairman of the Armed Services Committee next month.

Frist "will definitely seek the nomination of our caucus," Warner said after meeting with Frist at the headquarters of the National Republican Senatorial Committee.

Frist's announcement opens the door for an intense and potentially nasty internal fight for control of the party's Senate caucus. Lott quickly issued a statement indicating he would not yield.

"Senate Republican Leader Trent Lott will be the majority leader in the next Congress," a Lott spokesman declared in a statement last night. "He has a track record of loyalty, dedication and experience in shepherding President Bush's agenda for all Americans."

Frist backers

At least four other Republicans appear to support Frist. Warner said Sens. Don Nickles, Lott's deputy, and James M. Inhofe, both of Oklahoma, back the Tennesseean.

So does Sen. George F. Allen of Virginia, who is set to succeed Frist next month at the helm of the party's campaign committee.

Sen.-elect Lamar Alexander of Tennessee, who worked closely with Frist to capture the state's other Senate seat last month, issued a statement last night saying he would back him.

"If Bill Frist is a candidate for majority leader, I'm for him," Alexander said. "He's my neighbor, my friend, my senior senator and one of our best national leaders. These are serious times for our country. The Republican Party has a rare opportunity for leadership."

Frist's allies would not speculate on the number of senators lined up against Lott, whose talents for cutting political deals and counting votes are legendary.

Lott and his supporters pointed to his record of tending to the needs of his Republican colleagues and navigating the party's tough legislative battles.

But his opponents said the contest transcends the question of who is loyal to whom, and turns on what is best for the future of the Republican Party.

"I don't think any of us know how many votes Trent Lott has," Warner said.

But, he added, "This is not about votes. This is about what's best for the United States of America. How can we best support a strong president making daily tough decisions, and how best can we help preserve the credibility of the United States Senate?"

Dwindling support

Lott has lost the backing of some of his colleagues amid an outcry over comments he made this month that have been called racially insensitive.

His support among the Senate's 50 other Republicans continued to dwindle yesterday, as some colleagues expressed doubts about his ability to be effective, and many others declined to endorse him.

But Lott and key allies showed no signs of suspending their push to secure commitments from senators to stand behind the Mississippian.

Senate leaders have called a meeting for Jan. 6 to discuss Lott.

His remark Dec. 5 that America would have been better off if it had elected Strom Thurmond as president in 1948 - when he ran as a segregationist candidate - sparked a racially charged furor that could threaten Republican efforts to appeal to minority voters.

After Frist's announcement, aides said they believed that the leadership contest would be resolved well before the scheduled meeting next month.

The number of Republican senators expressing serious reservations about keeping Lott as their leader has mounted this week. Inhofe told radio stations in his home state that Lott's "ability as a leader dissipates on a daily basis."

He estimated Lott's chances of being majority leader at 30 percent.

Another conservative Republican, Sen. Craig Thomas of Wyoming, said in Gillette, Wyo., that he would be open to a new leader.

"I don't have a strong feeling about the personality that's there," Thomas said. "I don't condone what he did, and I'm not opposed to change either."

'Weakened' Lott

Nickles, also a staunch conservative, on Sunday became the first Republican to say his party should consider replacing a "weakened" Lott.

On Wednesday, moderate Sen. Lincoln Chafee of Rhode Island became the first in his party to say explicitly that he believed Lott should step aside.

Yesterday, Lott stayed mostly clear of the public spotlight after 10 consecutive days of media scrutiny and press appearances, both planned and spontaneous.

But he and his Senate supporters, led by Sen. Rick Santorum of Pennsylvania, were burning up the telephone lines, aides said, gauging support and trying to determine what it would take for Lott to survive.

He has good reason to hold out as long as he can. The next two weeks would give him ample time to do what is widely known as his forte: cutting deals with colleagues and counting votes to gauge his support.

Secretive rituals

Leadership races in Congress are highly secretive rituals in which personal relationships and commitments - legislative and otherwise - can trump all else.

Even some who question Lott's ability to lead the party in light of his crisis say his 30 years in Congress could serve him well in his campaign to round up the votes needed to save his leadership spot.

"He's horse-trading, and he's good at it," said one Republican strategist, speaking on condition of anonymity. "You're going to have dams moved from one state to the other. You're going to have roads built.

"He's talking to these members, he's reminding them what he's done for them, and he's telling them what he can do for them."