Published: Nov 21, 2013 7:24 AM EST

WASHINGTON (AP) - The Senate is nearing a potential showdown on curbing the power that the Republican minority has to block President Barack Obama's nominations, as Democrats edge toward muscling a rewrite of filibuster rules through the chamber.
    
Senate Majority Leader Harry Reid, D-Nev., was expected to force a vote as soon as Thursday on requiring only 51 votes to end filibusters, or delaying tactics, against nominees for high-level judgeships and agency officials.
    
Currently it takes 60 votes to end filibusters. That is a tough threshold for Democrats to achieve with their 55-45 edge over the GOP and in today's prickly political climate.
    
The change would be the most far-reaching to filibuster rules since 1975, when a two-thirds requirement for cutting off filibusters against legislation and all nominations was eased to today's three-fifths, or 60-vote, level. It would deliver a major blow the GOP's ability to thwart Obama in making appointments, though Republicans have promised the same fate would await Democrats whenever the GOP recaptures the White House and Senate control.
    
The clash was approaching as Democrats have grown increasingly irritated by the GOP's derailing of Obama's selections for top jobs, including three picks for pivotal judgeships in recent days.
    
"They have decided that their base demands a permanent campaign against the president and maximum use of every tool available," Sen. Jeff Merkley, D-Ore., a leading advocate of revamping filibuster rules, said Wednesday of Republicans. He said that consideration "is trumping the appropriate exercise of advice and consent" by GOP senators.
    
Republicans say they are weary of repeated Democratic threats to rewrite the rules. They say Democrats similarly obstructed some of President George W. Bush's nominees and argue that the D.C. Circuit's caseload is too low, which Democrats reject.
    
"I suspect the reason they may be doing it is hoping Republicans overreact, and it's the only thing that they could think of that would change the conversation about Obamacare," said Sen. Lamar Alexander, R-Tenn., using the nickname for Obama's troubled health care law. "But we're not that dumb."
    
Reid planned to use a procedural move called the "nuclear option" that would allow him to change the filibuster rule with just 51 votes, meaning Democrats could push it through without GOP support. Senate rules are more commonly changed with 67 votes.
    
Reid's proposal would not apply to legislation or Supreme Court nominees, aides have said.
    
Nomination fights are not new in the Senate, but as the hostility has grown the two sides have been edging toward a collision for much of this year.
    
The latest battle is over Obama's choices to fill three vacancies at the U.S. Court of Appeals for the District of Columbia Circuit. Since Halloween, GOP filibusters have derailed the president's nominations of District Judge Robert L. Wilkins, law professor Cornelia Pillard and attorney Patricia Millett for those jobs, which are lifelong.
    
The D.C. Circuit Court is viewed as second only to the Supreme Court in power because it rules on disputes over White House and federal agency actions. The circuit's eight judges are divided evenly between Democratic and Republican presidential appointees.
    
Senior Democrats wary of future GOP retaliation until recently opposed the move, but growing numbers of them have begun lining up behind Reid's effort.
    
In addition, two dozen groups, including the AFL-CIO and Sierra Club, wrote lawmakers Wednesday supporting the rules change because "rampant, ideology-based obstructionism is the new norm in the U.S. Senate."
    
This summer, Democrats dropped threats to rewrite Senate rules after Republicans agreed to supply enough votes to end filibusters against Obama's selections to head the Environmental Protection Agency, the Labor Department and other federal agencies.
    
On Wednesday, Sen. John McCain, R-Ariz., who helped fashion this summer's bipartisan agreement, met with Reid to discuss ways to avert the collision, according to aides from both parties who spoke on condition of anonymity because they were not authorized to discuss the session by name.
    
Neither side reported progress.

(Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)