As Muammar al-Qaddafi’s soldiers and mercenaries closed within 100 miles of the Libyan rebel stronghold of Benghazi Thursday, preparing to crush the revolution that just three weeks ago appeared on the brink of ending Qaddafi’s four decades of authoritarian rule, a top foreign policy aide to President Obama endured a withering barrage of criticism on Capitol Hill over the administration’s handling of the Libyan crisis.

Under Secretary of State for Political Affairs William J. Burns, the third highest-ranking official at the State Department and a veteran of high-level Washington across the last two presidencies, remained calm and collected under Thursday morning’s assault. He maintained that the president and Secretary of State Hillary Clinton have responded with urgency to the fluid and rapidly evolving events on the ground in Libya.

But lawmakers from both parties on the Senate Foreign Relations Committee openly expressed skepticism about those claims, with a bluntness rare in the normally staid confines of Capitol Hill hearing rooms.

None was more direct or impassioned than Sen. Marco Rubio, R-Fla., the freshman elected last year with Tea Party backing.

“The United States, quite frankly, looks weak in this endeavor,” said Rubio. “It looks unwilling, and maybe even unable, to act in this capacity….What are we going to do if there’s a bloodbath after this? The president of the United States has specifically said Qaddafi must go, but has done nothing since saying that, except have internal debates about it for a week-and-a half or two.”

When Burns cited pending efforts at the U.N. Security Council, where the U.S., France and Great Britain were working to secure Russian and Chinese agreement on a resolution establishing a no-fly zone over Libya, to be coordinated with Arab participation, Rubio —in extraordinarily blunt terms – challenged both the rationale and timing for that course of action.

“So our message to the dissidents,” Rubio said, “the people with the bravery to stand up to Muammar Qaddafi, and then the people maybe thinking to stand up to the Iranian regime, and in other places, our message is: ‘You guys go ahead and do this stuff, and if we can ever get the Russians or the Chinese to ever come around, we may or may not join you’?

“Russia and China don’t care about this stuff,” Rubio continued. “They don’t care that Muammar Qaddafi is going to massacre people. So if Russia doesn’t care, and China doesn’t care, and we care but won’t do anything about it, who is it up to – the French?”

Senate Democrats were less pointed in their comments, but expressed similar concerns about the Obama administration’s handling of the crisis. At one point, Sen. Robert Menendez, D-N.J. lamented all that the international community “said but didn’t do” about the Qaddafi regime’s military assault, and wondered aloud whether the president’s national security team was ever “serious about trying to shape the outcome” of the Libyan conflict.

“I read the statements [from administration officials] and I almost get a sense it's like a Texas two-step,” Menendez said. “I'm still not sure what we are supporting. It seems to me that it is a dangerous proposition to urge people to seek democracy and revolt and then basically not to help them. And so, you know, I am concerned as I listen to your answers, including what happens if Qaddafi prevails…I think we're going to miss an opportunity to promote democracy with a small 'd' throughout the region, and to be seen on the side of those who have aspirations of that.”

Burns conceded that “extremist fighters” could try to take advantage of a Qaddafi victory, and envisioned other, equally adverse, consequences that could flow from such an outcome. “The dangers of [Qaddafi] returning to terrorism and violent extremism himself,” Burns listed, adding to them “the dangers of the turmoil that he could help create, at a very critical moment, elsewhere in the region.”

Even after that dire assessment, Sen. James Webb, D-Va., suggested that Burns, an experienced diplomat, was putting the best face possible on an unpleasant portrait. “Your testimony sounds, frankly, optimistic,” Webb said, “which, after four years on this committee, I know that's how administration figures tend to sound. ‘Reform, peace, prosperity.’ But we've both worn enough hats to know that in this region [the Mideast], there's other forces at work.”

The bumpy reception accorded Burns ranked the hearing among the most contentious in recent memory. In July 2007, then-Attorney General Alberto Gonzalez faced angry queries from members of the Senate Judiciary Committee, including Sen. Herb Kohl, D-Wis., who memorably asked the witness: “Would you please explain to us why the administration of justice and the American people would not be better served by somebody sitting in the office who does not have all of the problems that you possess with respect to believability, credibility, confidence, trust?"

More recently, Treasury Secretary Timothy Geithner endured bipartisan criticism during a January 2010 hearing held by the House Committee on Oversight and Government Reform, which was examining the various bailouts the federal government had sponsored during the financial crisis. “You gave lame excuses then, you are giving lame excuses now,” thundered Rep. John Mica, R-Fla. “Why shouldn't we ask for your resignation as secretary of the Treasury?"

One of the few holdovers from the Bush administration to occupy a top spot on the Obama national security team, Burns has held the No. 3 position at State since May 2008. Prior to that, he served three years as the U.S. ambassador to Russia. He spent the first Bush term, from 2001 to 2005, as assistant secretary of state for Near Eastern Affairs, the bureau that oversees the Mideast.

At all points, Burns, a tall, slender man with a bushy gray moustache and a professorial disposition, refused to rise to the lawmakers’ bait. “We're still in the process to try to develop as full a picture as we can,” he said when Sen. Bob Corker, R-Tenn., asked whether the United States knows who the Libyan opposition is and what they stand for.

Under Rubio’s sustained assault, Burns calmly defended the administration’s pursuit of a fresh U.N. Security Council resolution against Russian and Chinese opposition. “I’m not assuming it’s going to fail,” Burns said. And when the Floridian asked if the administration has a backup plan, Burns replied dryly: “Senator, we've thought through lots of possibilities.”