If this Democratic Senate intends to maintain a majority, they need to accomplish things -- and they can't, under the current rules. Will they bite the bullet today?

Senator Harry Reid of Nevada, the majority leader, is poised to move forward on Thursday with a vote on what is known on Capitol Hill as the “nuclear option,” several Democrats said. Mr. Reid and the senators who have been the most vocal on stopping the Republican blockade of White House nominees are now confident they have the votes to make the change.

“We’re not bluffing,” said one senior aide who has spoken with Mr. Reid directly and expects a vote on Thursday, barring any unforeseen breakthrough on blocked judges.

The threat that Democrats could significantly limit how the filibuster can be used against nominees has rattled Republicans. Senator John McCain, the Arizona Republican who has brokered last-minute deals that have averted a change to filibuster rules in the past, visited Mr. Reid in his office on Thursday but failed to strike a compromise.

Senator Charles E. Grassley of Iowa took to the Senate floor and denounced Democrats, saying that if they changed the rules, Republicans would consider them applicable to all judicial nominees, including those for the Supreme Court. Mr. Reid has said he supports keeping intact the minority party’s ability to filibuster controversial Supreme Court nominees.

“Apparently the other side wants to change the rules while still preserving the ability to block a Republican president’s ability to replace a liberal Supreme Court Justice with an originalist,” Mr. Grassley said.