WASHINGTON – A straight party line vote from the Senate Judiciary Committee on Supreme Court nominee Samuel Alito puts intense pressure on a handful of Democrats from states that voted for President Bush last year as the president’s second nominee to the High Court in less than a year heads to the full Senate today for debate.

So far, only one Democrat, Senator Nelson, of Nebraska, has vowed to support the 55-year-old federal Court of Appeals judge from New Jersey in a confirmation vote that could come as early as Friday. Mr. Bush’s first nominee, Chief Justice Roberts, garnered 22 Democratic votes in September, but many of those Democrats have vowed to oppose Judge Alito.

The most recent moderate Democrat to defect is Senator Nelson, of Florida. Mr. Nelson has voted for 215 of Mr. Bush’s 225 judicial nominees, but said yesterday that he will vote against Judge Alito because he stands to replace a perennial swing vote on the court, Justice Sandra Day O’Connor. Mr. Nelson said Judge Alito did not demonstrate that he would be the “centrist voice I believe this nation needs” to replace the retiring justice.

“What this party line vote does is put all those moderate Democrats on the spot,” the president of the State University of New York at Albany and the author of “The Oxford Companion to the Supreme Court of the United States,” Kermit Hall, said. “Now they will be seen even more fully as siding with the Bush administration and against their Democratic colleagues on the committee if they vote to confirm Alito.”

The pressure is coming from both sides, with Democratic activists upset that their party has backed off rhetorically from blocking Judge Alito with a filibuster and Republican activists airing commercials in red states such as North Dakota urging Democrats to vote for Judge Alito. A former Democratic minority leader of the Senate, Senator Daschle, of South Dakota, is thought to have lost to a Republican last year for opposing Mr. Bush’s lowercourt nominees.

Front and center are Senators Conrad and Dorgan, of North Dakota; Senators Pryor and Lincoln, of Arkansas; Senator Landrieu, of Louisiana; Senator Byrd, of West Virginia; Senator Johnson, of South Dakota, and Senator Bingaman of New Mexico. Another possible Democratic vote for Judge Alito is Senator Menendez, of New Jersey, whose state went for Mr. Bush’s opponent, Senator Kerry, a Democrat of Massachusetts, last year but who is nonetheless under pressure from Democrats at home to vote for a native son.

In a first-ever straight party line vote in committee for a Supreme Court nominee, 10 Republicans voted to advance Judge Alito and eight Democrats voted against. Democrats cited concerns about the future of abortion rights and what they described as Judge Alito’s deference to executive authority. Republicans said Judge Alito’s opponents had politicized the confirmation process and set a new standard for nominees. Both of President Clinton’s nominees to the High Court, Justice Breyer and Justice Ginsburg, received unanimous votes out of committee.

“If qualifications, integrity, fairness, and judicial philosophy were still all that mattered in this process, Judge Alito would have been voted out of this committee by unanimous vote as well,” Senator Cornyn, a Republican of Texas, said. “But the new rule is that any nominee who refuses to promise to impose a liberal agenda from the bench is subjected to, as one of his opponents called it, the ‘you name it, we’ll do it’ tactics of distortion and smear.”

Despite Republican protests of changing standards, liberal activists have voiced outrage that Democrats have backed away from a filibuster. Democrats will have the 40 votes they need to block a vote, but the majority leader of the Senate, Senator Frist, of Tennessee, has said he will invoke the so-called “nuclear option” to eliminate the option of judicial filibusters if that tactic is tried. A former president of NARAL Pro-Choice America, Kate Michelman, called Democratic opposition to Judge Alito “a watershed moment.”

“If Judge Alito’s troubling views on privacy, reproductive rights, and myriad other issues do not merit serious consideration of a filibuster,” Ms. Michelman said, “it’s hard to imagine what might warrant such action.”

Debate on the nomination is expected to last three days, though Democrats have hinted they could attempt to delay a vote until as late as early February. Republicans, who had scheduled a retreat for Thursday and Friday, signaled late last week that they are willing to force a vote over the weekend. President Bush is set to deliver his State of the Union address at the end of the month, and both sides are looking to supply or deprive him of an applause line on the confirmation of another Supreme Court justice.