Chuck Hagel’s halting and unconvincing appearance Jan. 31 before the Senate Armed Services Committee hardened Republican opposition to his confirmation, but his critics would likely have to mount an unprecedented, and very divisive, challenge to block him from becoming Defense secretary.

Since Hagel appears to enjoy the support of most, if not all, Democrats, Republicans would have to filibuster his nomination — something that has never been done to a Cabinet nominee since the advent of the 60-vote threshold nearly four decades ago, according to Senate records.

Several Cabinet nominees have failed to win the backing of a majority of senators — and others have withdrawn their names before reaching the Senate floor — but a filibuster would mark a serious breach in the unwritten protocol that governs the Senate. Such a challenge could also disrupt the deal reached last month between Democratic and Republican leaders to overhaul the filibuster.

No Republicans have said yet that they will demand Hagel clear that 60-vote hurdle, but the possibility has been bubbling below the surface in the Senate in recent days.

An aide to Senate Minority Whip John Cornyn of Texas, who has been among the most vocal opponents of Hagel’s nomination, said Feb. 1 that “all options are on the table.”

Others in GOP leadership do not want to break with precedent. Senate Republican Conference Vice Chairman Roy Blunt of Missouri said Feb. 1 that while he does not support Hagel’s nomination, he also does not believe Republicans should filibuster it.

“I think for somebody who is going to be there the length of time the president serves, as opposed to a Supreme Court judge, that a majority in the Senate should be able to confirm,” Blunt told MSNBC. “I wouldn’t intend to be part of that majority, but my strong inclination would be that this is a vote that should be done by a majority and not a 60-vote standard.”

Top aides insist there is no discord among leaders, but statements made in the wake of Hagel’s highly scrutinized appearance before the Senate Armed Services Committee indicate there could be a difference in opinion. The consensus among leadership aides, however, is that if a filibuster is to happen, it likely would be staged by a junior member.

Freshman Republican Sen. Ted Cruz of Texas made clear at the hearing last week that he expects Hagel to provide the committee with additional transcripts from recent speeches, as well as more detailed financial disclosures.

Two sources speculated that Cruz was considering a hold. In response to a question on whether he would filibuster Hagel’s nomination, Cruz said in a statement that “we should do everything possible to ensure the Senate has the information necessary to make an informed decision.”

Sen. Lindsey Graham, an Armed Services Committee member, threatened to hold up Hagel’s nomination until the current Defense secretary, Leon E. Panetta, testifies before the panel on the attack last year on a U.S. diplomatic facility in Benghazi, Libya. Four Americans, including the U.S. ambassador to Libya, were killed.

That hearing could take place as early as this week and would likely pacify the South Carolina Republican, who said last week that he was as “happy as a clam” that a hearing was in the works.

Senate Minority Leader Mitch McConnell, R-Ky., has not taken a public position on either Hagel’s nomination or whether his caucus should filibuster the confirmation vote. But McConnell also aspires to one day be the leader of the majority party, under a GOP president, and if that scenario ever were to take place, it’s unlikely he would want to set the precedent now to make his life more difficult in the future.

Other aides made the practical case that Democrats and the White House seem confident they can pick up enough support to exceed the 60-vote threshold, which would make a cloture vote a tedious exercise in delaying the inevitable.

After a hearing that spanned more than eight hours and raked over Hagel’s record as a two-term Republican senator and his policy stances since he left Capitol Hill in 2009, several Republicans formally came out against him.

Besides Blunt, Republican Sens. Richard Burr of North Carolina and Mark S. Kirk of Illinois announced Feb. 1 that they would vote against Hagel. Two Republicans, Thad Cochran of Mississippi and Mike Johanns of Nebraska, have said they would vote for confirmation.

Even so, the White House expressed optimism.

“I would be stunned if, in the end, Republican senators chose to try to block the nomination of a decorated war veteran who was once among their colleagues in the Senate as a Republican,” White House Press Secretary Jay Carney told reporters on Feb. 1.

Hagel’s next step is to make it through a vote of the Senate Armed Services Committee, likely this week. But that requires only a simple majority, and Democrats outnumber Republicans on the panel 14 to 12.

Assuming he survives the committee vote, a GOP move to block Hagel on the floor has political risks that go beyond the nominee himself.

Just last month, senators reached a bipartisan deal to enact a package of modest changes to the Senate’s rules and procedures.

While the package preserves the right to force a 60-vote supermajority vote on bills and nominations, the goal of the deal — which was brokered by Majority Leader Harry Reid, D-Nev., and McConnell — is to allow business to progress more rapidly through the chamber.

It would set a new precedent for Cabinet nominees. Senators have traditionally given presidents wide latitude to pick their closest advisers.

Indeed, the chamber has rejected only nine nominees for Cabinet posts, the last one being in 1989 when the Senate defeated former Sen. John Tower to be Defense secretary. He received support from only 47 senators.

Steven T. Dennis contributed to this report.