Senate Republican leader Mitch McConnell is threatening to filibuster Chuck Hagel’s nomination to become Defense Secretary.

If he goes through with it, a move unclear to have enough Republican support to be successful, it would be historic. No cabinet-level nominee has been rejected because of a filibuster, and only two have been required to attain the 60 votes necessary to overcome a filibuster -- Dirk Kempthorne, President George W. Bush's nominee for Interior Secretary in 2006 and C. William Verity, Ronald Reagan's choice for Commerce Secretary. Both were easily overcame the filibuster, 85-8.

“It’s unclear yet,” McConnell told reporters in Kentucky when asked if his party would attempt a filibuster, forcing Hagel to need 60 votes for nomination. Currently, Democrats control the chamber with 55 seats.

“Sen. Hagel did not do a very good job before the Armed Services Committee this week. I think the opposition to him is intensifying. Whether that means he will end up having to achieve 60 votes or 51 is not clear yet.”

Cloture, the vote to end debate and overcome a filibuster, was withdrawn in the case of President's Obama's pick to be Labor secretary, Hilda Solis, as well as President George W. Bush's nominee to be EPA administrator Michael Leavitt. It was also "vitiated" for Rob Portman to become U.S. Trade represenative under Bush, according to the Congressional Research Service.

Plenty of others have been thwarted and one was even voted down since 1975 when the filibuster, requiring 60 votes, was introduced, according to lists provided by Don Ritchie, the Senate historian. That was George H.W. Bush’s Defense Secretary nominee John Tower, a former senator from Texas, who was accused of womanizing (he was married twice) and drinking to the point of alcoholism (he admitted he “drank to excess,” but, “I wouldn’t say I had a problem.”).

Before the Senate vote on his nomination, Tower even went on TV vowing not to drink as Defense Secretary or he would resign.

“Let me state that I have never been an alcoholic or dependent on alcohol,” Tower said in a statement distributed to the press and signed by his doctor. “I hearby swear and undertake that if confirmed, during the course of my tenure as secretary of defense, I will not consume beverage alcohol of any type or form, including wine, beer or spirits of any kind. I think I’d be obliged to resign if I broke the pledge. I’ve never broken a pledge in my life.”

In the end, Tower’s nomination was defeated 53-47. Ironically, the men leading the charge for and against Tower -- former Sens. John Warner (R-VA) and Sam Nunn (D-GA), respectively -- flanked Hagel at his confirmation hearing to urge his confirmation.

“There have been cabinet nominees rejected by the Senate, where there was a vote taken and gone down to defeat,” Ritchie said. “Some were withdrawn by the president because of opposition.”

Other, non-cabinet-level nominees were not able to overcome a filibuster, including John Bolton to be ambassador to the United Nations, a position President Obama now considers "cabinet-level."

There have been nine people nominated for a cabinet post who were voted on and formally rejected by the U.S. Senate, according to the Senate Historical Office. Tower’s, however, was the first since 1959 when Dwight Eisenhower’s  pick to be Commerce secretary, Lewis Strauss, failed 49-46.

Prior to that, just one other person was rejected in the 20^th Century. In fact, Charles B. Warren, Calvin Coolidge’s nominee to head the Justice Department has the distinction of being the only person in history rejected twice, first by a 41-39 vote. Coolidge then renominated him two days later only to see Warren rejected again by an even wider 46-39 margin just four days after being renominated.

The other six rejected nominees were all in the 19^th Century.

Some nominees have been withdrawn by presidents. There have been 21 nominees withdrawn by presidents, most recently former Sen. Tom Daschle, who was President Obama’s pick to head Health and Human Services in 2009; Bernard Kerik, George W. Bush’s nominee for Homeland Security in 2004; and Linda Chavez, Labor, under Bush in 2001.

President Bill Clinton withdrew three nominees -- for attorney general, director of the CIA, and Veterans Affairs.

Prior to that, a nominee hadn’t been withdrawn since Lyndon B. Johnson’s administration in 1969.

Whether Republicans would have the votes to successfully filibuster Hagel and prevent his confirmation is an open question, however.

So far, every Democrat, plus two Republicans (Thad Cochran of Mississippi and Mike Johanns of Nebraska) seem likely to vote for Hagel, giving him 57 votes.

Plus, so far, Republicans Roy Blunt of Missouri and John McCain of Arizona have said they oppose Hagel, but would not support a filibuster to block an up-or-down vote on his confirmation.

McCain had one of the sharpest exchanges of Hagel’s confirmation hearing last week before the Armed Services Committee over the surge in Iraq. (Hagel and McCain, both Vietnam veterans, had been friends before Hagel opposed the surge proposed by McCain.)

Despite that exchange, McCain told Politico Monday, “I just do not believe a filibuster is appropriate, and I would oppose such a move. … I will try to make that argument to my colleagues.”

CORRECTION: An earlier version of this post incorrectly stated, based on information from the Senate Historian's Office, that no cabinet-level nominee had been formally filibustered. There were two prior cases, however, in which cloture was invoked and there was a vote.