The U.S. Senate's top Democrat attacked Republican senators Thursday for blocking Chuck Hagel's nomination to be President Barack Obama's next secretary of defense, saying it was “shocking” and “tragic” that they would attempt such a move at a time when the U.S. military is engaged in so many places around the world.

The vote on Hagel was expected Friday, but Senate Republicans want more information about what Obama was doing on the night of the deadly attack on the U.S. diplomatic mission in Benghazi, Libya, in September. The attack killed the U.S. ambassador.

Senate Majority Leader Harry Reid said such a move is unprecedented. He said Republicans notified him Wednesday night they would withhold the votes needed to advance Hagel's nomination. That strategy would prevent Hagel's nomination from getting the required 60 votes.

The White House responded on Wednesday to the Republicans' request for information about Benghazi, Reid said. “But now Republicans say this is not enough,” Reid said. “This is no way to operate.”

Reid filed a motion Wednesday to limit debate and force a full Senate vote.

The White House on Thursday said Obama did not speak to any Libyan government officials until the night after the attack. White House counsel Kathryn Ruemmler sent a letter Thursday to three Republican senators saying former Secretary of State Hillary Clinton called Libyan President Mohamed Magariaf on Obama's behalf on Sept. 11 to coordinate additional support to protect Americans in Libya.

Ruemmler said Obama spoke to Magariaf on the evening of Sept. 12.

A White House official said there was no new information in Ruemmler's letter. The fact of Clinton's call to Magariaf has previously been public. If there were a need to push the Libyans to do something, Obama would have called, but the Libyans were trying to do the right thing and were being as helpful as possible, the official said.

The official, discussing internal communications only on the condition of anonymity, said that it wasn't clear that an earlier call from Obama to the Libyans would have been helpful in the deadly, fast-moving assault.

Hagel, a former Republican senator and twice-wounded Vietnam combat veteran, has already faced sharp questioning from his former colleagues over his past statements and votes on Israel, Iran, Iraq and nuclear weapons.

A bitterly divided Armed Services Committee on Tuesday voted to approve Hagel by a 14-11 vote, with all the panel's Democrats backing him. The committee's Republicans were unified in opposition to their onetime colleague, who will succeed Defense Secretary Leon Panetta if he's confirmed.

Panetta on Thursday said the struggle over Hagel is getting on his nerves. “The second-best Valentine's Day present would be to allow Sylvia and I to get the hell out of town,” he quipped at a Pentagon award ceremony for former Secretary of State Hillary Rodham Clinton.

While Democrats hold a 55-45 edge in the Senate and have the numbers to confirm Hagel on a majority vote, they need the support of five Republicans to clear the way for an up-or-down vote on him.

Sen. Lindsey Graham, a Republican, said he'll vote against ending debate on Hagel's nomination, and Sen. John McCain, another Republican who most sharply questioned Hagel during his Senate hearing, may join him.

They want to know whether Obama spoke to any Libyan government official during the assault and requested assistance for the American personnel at the mission. U.S. Ambassador Chris Stevens and three other Americans died in the raid at the compound in Benghazi.

“There seems to not be much interest to hold this president accountable for a national security breakdown that led to the first ambassador being killed in the line of duty in over 30 years,” Graham said. “No, the debate on Chuck Hagel is not over. It has not been serious. We don't have the information we need.”

McCain declined to say Wednesday whether he would try to delay Hagel's confirmation if Obama did not provide an answer. “My position right now is I want an answer to the question,” he said.

The nomination of John Brennan as CIA director is also being delayed; the Senate Intelligence Committee is pushing off a vote amid demands that the White House turn over more details about drone strikes against terror suspects and about the Benghazi attacks. Intelligence Committee Chairwoman Dianne Feinstein, a Democrat, said a vote likely will be postponed till late February.

A president's pick for a cabinet post usually requires only a majority vote, leading Reid to accuse Senate Republicans of orchestrating a filibuster against a nominee for defense secretary for the first time in the country's history.

But the top Republican on the Senate Armed Services Committee challenged Reid's claim, saying it's not unusual to hold a cabinet nominee to a 60-vote threshold. “It's not a filibuster,” said Sen. James Inhofe. “This has happened (before), and it's happening again right now.”

Sen. Carl Levin, the Democrat and chairman of the Armed Services Committee, said he's confident that the White House will supply the information Graham and McCain want and that Hagel will be confirmed. (AP)