The White House Thursday demanded a vote to confirm President Barack Obama's pick for Pentagon chief this week and blasted Republicans for sending a bad signal to US allies by delaying his appointment.
As a political standoff escalated, a top Democrat told AFP that his party may not have the votes to drive through Hagel's nomination as defense secretary before the Senate goes into a week-long recess.
White House spokesman Joshua Earnest said the delay had jeopardized Hagel's planned appearance at a NATO ministerial meeting next week in Brussels, with NATO powers to discuss the pull-out from Afghanistan.
"It is difficult to explain to our allies exactly why that is happening. It also sends a signal to our men and women in uniform," Earnest said.
"We need our new defense secretary to be there," he said, branding the Republican procedural tactics "unconscionable."
Before allowing a vote, Republicans had demanded more information on Hagel's finances and on Obama's role in responding to an attack on the US consulate in Benghazi, Libya last year.
Senate Democratic majority leader Harry Reid responded by trying to force a vote on the nomination, but will need five Republicans to get to the 60 votes needed to overcome the blocking tactics.
Without such a filibuster, presidential nominations require a simple majority of senators in the 100-seat chamber -- a barrier Hagel is almost sure to surpass.
Reid accused Republicans of acting like children over Hagel, saying it was "shocking" they would do so while the United States was at war.
"The filibuster of senator Hagel's nomination is unprecedented," Reid said. "For the sake of our national security, it's time to put aside this political theater -- and that's what it is."
Carl Levin, chairman of the Senate Armed Services Committee, told AFP that it was unclear whether Hagel would get the 60 votes this week.
"We don't know ... you're never sure until the vote," said Levin.
Hagel's hopes of swift confirmation appear to rest with his one-time friend and colleague Senator John McCain, who eviscerated him in a confirmation hearing and has not yet made clear whether he would support a filibuster.
McCain's fellow Republican hawk Senator Lindsey Graham suggested that he would drop his delaying tactics only after the Senate returns from a week-long recess in the week of February 25.
Both men Thursday got a letter from the White House with new details of Obama's actions on the night of the attack by militants on the consulate in Benghazi, which killed four people, including US ambassador Chris Stevens.
It confirmed that then secretary of state Hillary Clinton had called Mohammed Megaryef, president of the Libyan national assembly, on the night of the Benghazi attack but Obama had not.
Graham had argued that the extra weight of a demand by the US president, at a time when the consulate was under siege, might have mobilized Libyan forces more quickly and saved some American lives.
According to the letter from White House counsel Kathryn Ruemmler, Obama spoke to Megaryef on the night after the attack.
The letter prompted Graham to say he might drop resistance to a confirmation vote after the Senate break, a move which would clear the path for Hagel.
But he added a qualifier, saying "unless there's some bombshells, unless he, (said) that the Israeli government is bloodsucking vampires or something like that, I got a feeling I'd be ok."
Hagel was picked to replace outgoing Pentagon chief Leon Panetta at a time of rising tension in Middle East hotspots, budget battles at home and after this week's rogue nuclear test by North Korea.
Panetta spent his last day at work on Thursday before returning to his home in California, though will formally retain the responsibilities of Defense Secretary until his successor is confirmed.
Several Republicans have taken issue with Hagel's public stances in recent years on Iran, nuclear weapons, Israel and the US troop surge in Iraq.
They have demanded financial compensation data and transcripts from speeches Hagel gave to foreign audiences, saying some records are missing.