“These are dark days in the history of the Senate,” Mr. McConnell said, his voice at times quivering with anger. If Mr. Reid moved ahead with a rules change, Mr. McConnell added, he would go down as “the worst leader of the Senate ever.”

Senators from both parties planned to continue talks in the coming days to avert the rule change, which is so controversial that it is nicknamed “the nuclear option.” Any altering of the long-held Senate custom of the filibuster is a rare and, some argue, perilous move as one year’s majority party could be the next year’s minority.

Mr. Reid said he would back away if Republicans confirmed seven pending nominees, including leaders for the Labor Department, the Environmental Protection Agency, a consumer protection agency and vacancies to a politically important labor law oversight board. And it is possible that a bargain could be reached similar to the one struck by the “Gang of 14” in 2005 when Democrats were blocking President George W. Bush’s judicial nominees. That deal defused a similar standoff.

Top members of both parties said they remained uncertain whether Mr. Reid, who has been pressed for months by some Democrats to take action against Republicans, would follow through with his threats or was using the fight as leverage to force Republicans to change their ways.

But increasingly, the two sides appeared headed down irreconcilable paths.

The step that Mr. Reid has endorsed, which he said all but two or three Democratic senators support, would not affect filibusters of legislation or judicial nominees. But it would prevent Republicans from being able to require a supermajority of 60 votes for the confirmation of presidential appointees to cabinet-level or other executive branch positions.