WASHINGTON – Senator Schumer said he thinks a partisan fight in the Senate is likely if President Bush elevates Justice Thomas of the Supreme Court to the position of chief justice, and the Democrat warned Republicans not to engage in “banana republic” tactics to push through their nominees.

The senior New York senator, who has been a leader in past efforts to block Mr. Bush’s judicial nominees, called the conservative judge “one of the few who has wanted to roll, roll back the clock.”

Mr. Schumer said he would support pro-life nominees for the bench as long as they were otherwise “mainstream” in their legal views.

“Judges on the far right or on the far left shouldn’t be put on the court because they feel so passionately that they want to make law,” Mr. Schumer said on ABC News’s “This Week with George Stephanopoulos.”

The Schumer warning comes as Senate Republicans prepare to meet this week to plot their strategy for what could be a tense battle over confirming Mr. Bush’s choices for the judiciary. The majority leader, Senator Frist of Tennessee, said they are considering a handful of options to end the possibility of filibusters, including a dramatic procedural change referred to as the “nuclear option,” in which Senate leadership would permanently change the rules to require only a simple majority 51 votes to end a filibuster directed against a judicial nomination, rather than the current 60 votes.

Mr. Schumer disparaged the move as befitting a “banana republic.”

“It really is Third-World-country like,” he said. “And of course, that would end any hope of comity. That would make the last Senate look like a very polite tea party.”

The executive director of Committee for Justice, a group that advocates for Mr. Bush’s nominees, Sean Rushton, questioned whether Senate Democrats would unite to fight the president’s nominees after an election in which issues involving values played an important role. “Chuck Schumer can say what he likes,” Mr. Rushton began, but he said Senate Democrats “from red states” might not “want to signal right out of the box a continued era of obstructing nominations.”

The Thomas nomination as chief justice of the United States has been discussed by Republicans “for some time,” Mr. Rushton said, and the judge would be a “good pick.”

He noted, though, that the scenario had the drawback of requiring two separate confirmation procedures: one to elevate an associate justice to replace Chief Justice Rehnquist, who is ill with thyroid cancer, and one for a new associate justice.

Like Dr. Frist, Senator Specter of Pennsylvania, the moderate, pro-choice Republican who is in line to replace Senator Hatch of Utah as chairman of the Judiciary Committee, would not rule out the “nuclear option,” saying it could help pressure Democrats to bend.

“We have to confirm federal judges. And the fact that it’s on the table may be a significant factor in pushing for a compromise,” Mr. Specter said.

Asked whether he would support Justice Thomas for chief justice, Mr. Specter declined to comment directly, saying only, “I do believe the president ought to have very substantial deference in his nominations.”

Mr. Bush said during his re-election campaign that if he has to fill a seat on the Supreme Court, he would appoint a judge who applies a “strict constructionist” approach to the Constitution. He has said he would like a nominee in the mold of Justices Scalia and Thomas, who have voted to overrule the court’s holding in Roe v. Wade, which recognized a constitutional right to privacy that protects most abortions.

Mr. Specter has been under fire from social conservatives for suggesting after the election that anti-abortion candidates could have a tough time getting confirmed by the Senate.

Yesterday, the Pennsylvanian continued his effort to reassure his colleagues that he has never applied an abortion “litmus test” to Supreme Court nominees and has supported all of Mr. Bush’s nominees in committee and on the floor.

“I have voted for pro-life nominees,” Mr. Specter said. “When Chief Justice Rehnquist came up for confirmation, he had already voted against Roe v. Wade. I voted to confirm him, voted to confirm Justice Scalia, Justice O’Connor, Justice Kennedy. When Justice Clarence Thomas came up, I led the fight to confirm Justice Thomas – almost lost my Senate seat in the process.”

Dr. Frist was noncommittal, however, about Mr. Specter’s ascension to the chairmanship.

He said the Pennsylvanian had “not yet” made a persuasive case that he should get the job. The majority leader suggested Mr. Specter must all but guarantee to his Senate colleagues that he will back all of the president’s nominees.

“I would expect Chairman Specter …if it’s Chairman Specter…to have a strong predisposition to supporting that nominee sent over by President Bush,” Dr. Frist said yesterday on “Fox News Sunday.”

Dr. Frist said he has three or four options to stop the filibusters, which he called “intolerable.”

Senator McCain, a Republican of Arizona, said he supports Mr. Specter for the chairmanship and predicted he would be confirmed.

“Obviously, he is not of the right wing of our party. But I believe that Arlen has done a good job, and… I think he’ll be confirmed,” Mr. McCain said on “This Week.”

Mr. McCain said he was concerned about permanently doing away with the rights of the minority in the Senate. “If I believed Republicans … would be in the majority forever, I’d be far more favorably disposed,” he said.

Also yesterday, Mr. Schumer declined to comment on reports that he is in negotiations with Senate Democrats to take a seat on the powerful Finance Committee and to head their fund-raising effort, a move which would require him to forgo a run for governor of New York in 2006.

“The only thing on my radar screen right now is being a good senator, delivering for the people of New York, and helping this country,” he said.