The US Senate will vote on Thursday on a motion to end debate on the nomination of Chuck Hagel as President Barack Obama's new secretary of defense.
If the motion passes, the vote would clear the way for the 100-member Senate to decide whether to confirm the former Republican senator as Pentagon chief, but it is expected to be narrowly defeated.
Some Republican senators said they want more time to consider Hagel's nomination.
Senator John McCain of Arizona said on the Senate floor that he believes there will be the 60 votes needed to stop debate - and clear the way for a vote by the full Senate on confirmation - after the Senate returns on Feb. 25 from next week's recess.
Chuck Hagel's nomination to be US defense secretary went into political limbo on Thursday as Republicans stepped up blocking tactics and Democrats accused them of putting the country at risk by delaying the filling of a major security post.
Senate Majority Leader Harry Reid made an impassioned appeal for Hagel's confirmation amid questions over whether he could get the 60 votes needed to overcome Republican roadblocks preventing a vote.
"This isn't high school getting ready for a football game. We're trying to confirm somebody to run the defense of our country," Reid said on the Senate floor after Republicans said they would try to block Hagel's confirmation.
The Senate is to consider on Friday whether to clear the way for the confirmation vote.
Sources said Republicans were in heated negotiations with the White House on a compromise under which at least a handful of Republicans would agree to let the confirmation go ahead. A major sticking point is the Obama administration's refusal to release more information about the deadly September attacks on US facilities in Benghazi, Libya.
The struggle over Hagel's nomination is one of many battles raging between Obama's Democrats and Republicans in Congress, including disputes over gun control, immigration rules and dealing with huge budget deficits.
Reid, a Nevada Democrat, accused Republicans of trying to score political points by coming up with one reason after another to delay confirmation of a new Pentagon chief. He said is was a shame Republicans were using the blocking tactic known as a filibuster for the first time ever to prevent a vote on a defense secretary nominee.
"For the sake of our national security it is time for us to put aside political theater, and that's what it is. People are worried about primary elections," Reid said.
If confirmed, Hagel, a decorated Vietnam War veteran and former Republican senator from Nebraska, would replace Leon Panetta, who is retiring. Panetta said he will not leave before his successor is in place, but has expressed eagerness to return to his home in California.
Democrats, who have united in support of Hagel, control 55 seats in the 100-member Senate and could confirm Hagel without any Republican backing. A Cabinet nominee requires the support of only a simple majority to be confirmed.
However, they need the support of 60 senators to clear the procedural hurdles and allow the vote.
Republican anger started with Iraq
Hagel broke from his party as a senator by opposing former President George W. Bush's handling of the Iraq War, angering many Republicans. Some Republicans have also raised questions about whether Hagel, 66, is sufficiently supportive of Israel, tough enough on Iran or capable of leading the Pentagon.
His performance at his confirmation hearing before the Senate Armed Services Committee also drew harsh criticism. Even some Democrats have said he appeared unprepared and at times hesitant in the face of aggressive questioning.
Earlier, two Republicans had said they would vote for Hagel and several others said they would oppose procedural hurdles, but those positions may have changed.
Republican Senator John McCain, for example, had said he opposed procedural tactics to block the vote on Hagel, but was reconsidering to press the White House to release more information on Benghazi.
A senior Senate Democratic aide said Republicans had informed Democratic leaders that there were not enough Republicans willing to join the Democrats to yield the 60 votes to allow the vote to go through.
Republicans insist that Reid brought the problem on himself by trying to rush Hagel's confirmation. Obama nominated Hagel on Jan. 7 and his hearing before the Armed Services panel took place on Jan. 31.
Democrats said a wait of two weeks for a vote after his hearing was not unusually short. They also noted that many of Hagel's most vocal opponents served with him during his two terms in the Senate from 1997 to 2009 and knew him well.
A White House spokesman said Obama still stands strongly behind Hagel, and said the "unconscionable" delay does not send a favorable signal to allies or US troops.
"The president stands strongly behind Senator Hagel," White House spokesman Josh Earnest told reporters on board Air Force One. "It does not send a favorable signal for Republicans in the United States Senate to delay a vote on the president's nominee, a nominee who is a member of their own party, to be Secretary of Defense."
The confirmation of another of Obama's national security nominees, John Brennan for CIA director, also faces a delay amid jockeying between the White House and lawmakers over the release of sensitive documents, including some related to Benghazi.