Six weeks out from Election Day, Republicans have found an opening to strike President Obama on foreign policy by using the administration's handling of a fatal attack in Libya to punch holes in what had been the president's strongest first-term achievements.

Top Republicans are hammering the administration for insisting that the Sept. 11 attack on the U.S. consulate in Benghazi, Libya, that killed four Americans was a spontaneous response to an anti-Muslim video, even though intelligence officials quickly understood it was a coordinated attack planned by a terrorist group.

"It was obvious ... this was a planned attack," Sen. John McCain, the top Republican on the Senate Armed Services Committee, told CBS News on Thursday. "Anybody who understands warfare knows that is not a spontaneous demonstration. ... They carried heavy weapons, mortars, RPGs."

Obama's strength on foreign policy had previously appeared unshakeable. He had wound down the wars in Iraq and Afghanistan and authorized the mission that killed al Qaeda leader Osama bin Laden. Polls show voters consistently rate Obama as more credible on foreign policy issues than his Republican opponent, Mitt Romney.

But Republicans now hope that the dynamic will shift as questions arise over what the White House knew about the attack and when it knew it.

The administration's explanation of the attack has been continuously changing. For two weeks, the White House insisted it was a spontaneous protest set off by the anti-Muslim video, similar to protests in Egypt. By Thursday, the administration was calling it a terrorist attack.

Republicans claim the administration was downplaying the role of terrorists in the attack to protect the president politically.

"How can President Obama address a crisis if he can't be honest about its causes?" wrote Republican National Committee Chairman Reince Priebus in an op-ed Thursday. "How can he fight terrorism if he won't identify terrorist acts?"

The White House denies it manipulated or hid information.

"Every step of the way, the information that we have provided to [reporters] and the general public about the attack in Benghazi has been based on the best intelligence we've had and the assessments of our intelligence community," said White House spokesman Jay Carney.

Romney initially stumbled in criticizing Obama over the Libya attack, and he hasn't explicitly addressed the issue since then. While some supporters are now encouraging Romney to be more aggressive in assailing Obama on foreign policy, others question whether that would help Romney with voters intently focused on the economy.

"I think the much bigger challenge for Gov. Romney in this regard is not to prove that the Obama administration lied to the American people, because that seems self-evident," said Danielle Pletka, vice president for foreign and defense policy studies at the American Enterprise Institute. "I think the bigger challenge is to make people care about foreign policy."

The Obama campaign is prepared for Romney to go on the offensive on foreign policy, noting Romney's own flubs during an overseas trip.

"We know that Mitt Romney wants to change the subject, wants to get at a strength for the president, and it's clear that that's what's going on here," said Obama spokeswoman Jen Psaki. But "every time Mitt Romney has attempted to dip his toe in foreign policy waters, it's been an unmitigated disaster."