WASHINGTON – Democrats yesterday threatened to shut down Senate business if Republicans deploy the so-called “nuclear option” in the war over judicial nominations, ending the ability of senators to filibuster judges.

Republicans are considering changing the chamber’s long-standing rules to reduce the number of votes needed to force a floor vote on judges from 60 to 50. That would mean the 44 Senate Democrats could not hold up votes on President Bush’s judicial nominees.

The minority leader in the Senate, Senator Reid, warned the majority leader, Senator Frist, in a letter yesterday that Democrats would bring Senate business to a standstill if Republicans made the move.

“The majority should not expect to receive cooperation from the minority in the conduct of Senate business” if the rules are changed, wrote the Nevada Democrat.

The ultimatum did not apply to legislation “vital to our troops or other national security interests,” or “critical government services,” but would otherwise affect even the most routine matters, he wrote.

“Democrats in the Senate may be in the minority, but we represent millions of American citizens. The nuclear option would deny these Americans their rightful voice in the governance of the nation,” he wrote to Mr. Frist in a letter he claimed spoke for all Senate Democrats. Senator Schumer stood with Mr. Reid for the announcement on the Senate steps, and is “in total support of it,” said his spokesman, Israel Klein. Senator Clinton did not comment yesterday.

Senator Kennedy of Massachusetts seconded the letter yesterday in a statement accusing the Republican majority of acting out of “desire for absolute power.”

But the majority leader did not betray concern about the Democratic threat.

“To shut down the Senate would be irresponsible and partisan. The solution is simple: return to 200 years of tradition and allow up or down votes on judges,” responded Mr. Frist in a statement. Mr. Frist’s office would not confirm whether or when Republicans would hold a vote on the subject.

A former aide to Mr. Frist, Manuel Miranda, who is active in coordinating the anti-filibuster effort, predicted the vote would come next month – “at the end of April,” he said.

In his letter, Mr. Reid said the Senate confirmed 204 judicial nominees and defeated only 10 nominations. He noted that the federal court vacancy rate reached its lowest level in 15 years. But a group that advocates for the nominees, the Committee for Justice, counts 20 judges whose nominations are being either filibustered or delayed.

Mr. Reid’s letter comes as Republicans face mounting pressure by conservative advocacy groups who believe they have sufficient Senate votes to end the filibuster quickly.

The Family Research Council has bought television ads in Nevada targeting Mr. Reid for his role in blocking judges. The group is now planning to run ads in Kentucky, the home state of the Republican whip, Senator McConnell.

“There is a comfort level that the votes are there, they just need a little encouragement to get it done,” said the vice president of government relations for the Family Research Council, Constance Mackey.

Mr. McConnell said yesterday that he supports the measure, which Republicans have begun referring to as the “constitutional option” or the “majority rule option.”

“This obstruction necessitates that we restore these norms and traditions, and that includes through the use of the so-called ‘constitutional’ option,” he said in a written statement.

Mr. McConnell added that the Democratic threat would “place at risk funding for child care, education, highway projects, an energy bill, and other important programs that impact the lives of every American.”

Other Republicans were quick to denounce Mr. Reid’s letter.

“It shows really how desperate they have become,” Senator Cornyn of Texas told a conference on judicial nominations at the Heritage Foundation yesterday. “They will make our case for us and see a serious backlash to what they are doing,” he said.

Some conservatives remain opposed to the nuclear option for a variety of reasons, including concerns the Democrats would pay a higher political price if they were allowed to continue their tactics.

The conservative commentator, George Will, told the Heritage Foundation audience that President Bush and his chief political adviser, Karl Rove, have credited Democratic obstruction of judicial nominees as contributing to Republican victories.

Mr. Will also warned Republicans that the rule change could lead to pressure to eliminate the filibuster in other contexts, such as blocking legislation or international treaties.

“You are sharpening a scythe that will in short order cut down all filibustering,” Mr. Will said.

But a former White House counsel to the first President Bush, C. Boyden Gray, who is chairman of a group that advocates for the current president’s judicial nominees, said the use of the filibuster against judicial nominees departs from tradition.

“What we are doing is not changing the rules, but restoring the status quo that existed for some 200-odd years,” he said.

A professor at William & Mary School of Law, Michael Gerhardt, said Republicans may rue the proposed rule change if they are ever back in the minority, since they themselves have used the filibuster against Democrats.

“Nobody in this debate over the filibuster has clean hands,” he said.