Democrats now control 55 Senate votes. Republicans have 45, but the GOP often still asserts itself by using the filibuster to keep nominations or legislation from coming to a vote.

Republicans charge that imposing the nuclear option would eliminate the minority party's chief means of keeping the majority in check and jeopardize any potential bipartisan agreements on top-priority legislation, including immigration reform, the budget and tax reform.

"The fact is, this poisons the well," said Mark Calabria, director of financial regulation studies at the Cato Institute and a former GOP Senate aide. "This ends the second term of the presidency in terms of any cooperation with the Senate."

Democrats countered that the use of filibusters must be blunted after Republicans breached a long-standing agreement not to block presidential nominees, particularly Cabinet appointees, and violated a deal struck in January that was supposed to reduce the time it took to confirm nominees. That kind of obstructionism is hindering government operations, Democrats said.

"The fact is," one top Democratic aide told the Washington Examiner, "a promise was made to us in January to operate under the norms and traditions of the U.S. Senate when it comes to nominees. But that hasn't been upheld. Instead, you have a whole branch of government that is being stopped from doing their jobs by a minority of the United States Senate."

Senate Minority Leader Mitch McConnell, R-Ky., said recently that any move by Reid to alter the filibuster would violate Senate rules, which require 60 votes to change those rules.

"Once you do that," McConnell said, "it changes the entire Senate into the House."

Reid threatened to abolish the filibuster after Republicans blocked three particular presidential nominees: Gina McCarthy, tapped to head the Environmental Protection Agency; Thomas Perez, Obama's labor secretary pick; and Richard Cordray, the acting head of the Consumer Financial Protection Bureau whose recess appointment by Obama still requires Senate confirmation.

McCarthy's nomination is in jeopardy largely because Republicans want to demonstrate opposition to Obama's new limits on coal-fired power plants that the GOP said would kill jobs and raise energy prices. Perez is accused of flouting the rule of law in pursuit of an ideological agenda. Cordray was targeted because the GOP wants the consumer bureau to be run by a bipartisan commission instead of a presidential appointee.

Liberal groups and labor unions are putting intense pressure on Reid to change the rules because, they said, Republicans are blocking Obama's nominees to the National Labor Relations Board, weakening the rights of workers. But political experts predict Reid won't actually impose the nuclear option, that he is using the threat to get Republicans to approve at least some Obama nominees.

Another obstacle to curbing the filibuster is opposition from some of Reid's fellow Democrats, who worry that the change could be used to marginalize them should they lose their narrow Senate majority.

"Reid probably does not have 51 votes to go nuclear and probably as majority leader prefers not to risk blowing up the Senate," said Sarah Binder, a congressional expert at the Brookings Institution.

Despite the threat, McConnell, the Republican leader, has doubts that Reid will go nuclear.

"I understand my friend the majority leader is under a lot of pressure," McConnell said. "I've known him for a long time and, deep down, I know he understands the far-reaching consequences of 'going nuclear.' I think he actually realizes how terrible an idea that would be."