Because Republicans are united in their dislike of the proposed changes, Mr. Reid would never get 67 votes — two-thirds of the Senate — to break a filibuster on the filibuster change. So he could instead avail himself of a controversial option that some proponents believe is available only on the first day of a new Congress and change those rules via majority rule, or 51 votes. Opponents insist that such a move would violate Senate rules.

A majority of Democrats, frustrated by what they say is the consistent and brazen abuse of the filibuster by Republicans, appear to support changes to the rules, and some believe they do not go far enough. But others, deeply aware that a majority party today can be the sad and lonely minority tomorrow, are not keen on playing the “nuclear option” card, with majority rule.

“I don’t like the nuclear option,” said Senator Bill Nelson, Democrat of Florida. “I reserve the right to decide later, but instinctively I don’t like it. It’s avoiding the rules.” Mr. Nelson added that “a body like this runs on comity and common sense,” and he said he worried that going nuclear would do serious damage to that atmosphere.

The divide exists somewhat along electoral generational lines. Newer senators, appalled by the molasseslike movement of bills and the overall dysfunction of the chamber, have been urging Mr. Reid to make the changes. Senator Tom Udall of New Mexico, who took office in 2009, has been especially ardent, though he gets a great deal of support from Senator Tom Harkin of Iowa, who has been laboring against the filibuster for decades.

Mr. Harkin said he was not confident that the Democrats had the votes they needed to jam the changes through. “There are some Democrats who don’t want to change the way we do things around here,” he said. Other Democrats said that while they did not believe the votes were secure, they felt certain that their colleagues would come along if Republicans and Democrats could not come to an accommodation by the end of the year.