WASHINGTON - Senate Republican leader Trent Lott is fighting to keep his leadership post after a harsh rebuke yesterday from President Bush and mounting criticism from members of both parties for his recent comment appearing to praise racial segregation.

The furor surrounding Lott's remark that the nation would have been better off had Sen. Strom Thurmond been elected president in 1948 - when Thurmond ran on a pro-segregation platform - grew hotter when Bush delivered a strong reproach of the Mississippi senator, saying his words "do not reflect the spirit of our country."

"Any suggestion that the segregated past was acceptable or positive is offensive, and it is wrong," said Bush, drawing loud applause from a racially diverse crowd in Philadelphia. "He has apologized, and rightly so."

Conspicuously absent from the president's stern comments was any hint of support for Lott, who has issued repeated apologies for what he said Dec. 5 at a 100th birthday party for Thurmond.

After a week of silence on the incident, the president, who has increasingly been focusing attention on his own re-election in 2004, appeared eager to distance himself from Lott.

"Every day that our nation was segregated was a day that America was unfaithful to our founding ideals," Bush said. "The founding ideals of our nation and in fact the founding ideals of the political party I represent was and remains today the equal dignity and equal rights of every American."

After initially saying that Bush would not take a position on whether Lott should forgo the position of majority leader, which he is to assume next month, White House officials said that the president does not believe Lott needs to step aside.

But Bush's public statement was widely interpreted on Capitol Hill as a signal that Lott's political situation had grown far more serious. Republican Sen. John McCain of Arizona said in a TV interview that "significant damage has been done" to Lott's standing.

"It's bad, and I think it's getting worse," one senior Republican aide said, speaking on condition of anonymity.

Senate Republicans are not inclined to oust Lott for now, the aide said, but "the variable here is the administration."

Some Republicans have long harbored doubts about Lott's leadership ability and negotiating skills. He has never been particularly popular with the business community, which is closely allied with the White House.

"He does have this problem of saying stupid things which play into the worst perceptions of the Republican Party," the GOP aide said. "If I were Lott, I would be building bridges to the Bush administration as fast as I could."

Adding to Lott's troubles yesterday were a steady stream of revelations - none of them new to those familiar with Lott's political career and background - about his past positions on racial matters.

Those included an article on Time magazine's Web site about Lott's efforts in the early 1960s to keep his college fraternity from admitting blacks, and reports about his push in 1981 to persuade the Supreme Court to uphold the tax-exempt status of Bob Jones University, which banned interracial dating.

Even his hometown newspaper, the Mississippi Press in Pascagoula, declared in an editorial that Lott "doesn't deserve to be majority leader."

"We encourage the Senate Republicans to replace him with someone with more progressive beliefs," the newspaper said.

Lott touched off the latest furor over his racial stances when he said that Mississippi was proud to have supported Thurmond's presidential bid in 1948, adding: "And if the rest of the country had followed our lead, we wouldn't have had all these problems over all these years."

After initially apologizing, in a written statement, for using a "poor choice of words," Lott said Wednesday that his remarks were "insensitive" and "terrible."

The Congressional Black Caucus called yesterday for a formal censure of Lott's words, calling his comments part of "a longstanding pattern of behavior that can no longer be ignored or tolerated."

"Anything less than a total condemnation will forever lead millions of Americans to believe that these remarks, and this continuing pattern of behavior, are condoned by the president, Congress and the Republican Party," the caucus said in a statement.

Rep. Elijah E. Cummings, the Baltimore Democrat who is the incoming caucus chairman, said: "The verbal offense that Senator Lott has committed is damaging to peace and unity in America."

While close allies rallied to Lott's defense, the strong words from Bush appeared to embolden some Republicans in Congress to come forward with their own harsh criticisms. Some demanded that Lott provide a better accounting of his positions on civil rights.

Freshman Sen. James M. Talent, a Missouri Republican, said: "Senator Lott's comments were wrong and insensitive." After speaking to Lott by telephone, he said, "I believe his apology is sincere and that he did the right thing by withdrawing his comments."

Sen. Olympia J. Snowe of Maine said she "was deeply disappointed with his choice of words that were insensitive and offensive." A leading Republican moderate, Snowe said she discussed the comments with Lott in a phone call this week and accepts Lott's apology.

"Nevertheless, I also believe it is essential that he forcefully convey to the nation and most especially the African-American community, in a public forum, what is truly in his heart as to the depth and breadth of his commitment to civil rights, and I have urged him to do so," she said.

Lott's office scrambled late yesterday to respond to those calls, issuing a five-page document outlining the Republican leader's achievements benefiting African-Americans, including his support for education initiatives and economic development, and a resolution he introduced condemning church arson.

Lott has remained away from Washington and out of public sight as the tempest over his comments has grown more fierce.

McCain was among those calling on Lott to speak directly to the American people in a more public way. He said Lott should have "a full-blown press conference with an opening description of his absolute outright hostility to discrimination in any form."