DENVER, Colorado — Mitt Romney and Republican allies launched a coordinated assault on President Barack Obama's foreign policy Monday, seeking to exploit a fissure ahead of the debut debate between the White House rivals.

The Republican nominee, reinforcing his campaign's argument that Obama has downplayed deadly violence in the Middle East, warned that the president's policies have "heightened the prospect of conflict and instability."

Obama "does not understand that an American policy that lacks resolve can provoke aggression and encourage disorder," Romney wrote in a Wall Street Journal op-ed.

"We're not moving (events in the Middle East) in a direction that protects our people or our allies. And that's dangerous."

As Romney flew to Denver, Colorado for the first of three face-to-face debates with the president, he enlisted running mate Paul Ryan to strafe Obama's Afghanistan policy.

Ryan said the White House made a "political decision" to draw down 22,000 troops in September, just weeks before the November 6 election, a move he warned would put the remaining US forces in greater danger.

"They're still fighting," Ryan told conservative radio talkshow host Laura Ingraham.

"We would never put politics ahead of what our commanders say is necessary to do the job and keep our soldiers as safe as possible when they're prosecuting this war."

On Sunday, Ryan revealed that Romney was planning a major foreign policy speech soon.

Two Republican senators also harshly condemned the administration, including Obama's 2008 White House adversary John McCain.

"It's unravelling all over," McCain told cable network MSNBC.

"Because of this failed national security policy, the chickens are coming home to roost in Iraq, in Afghanistan, in Libya and of course in Syria, which continues to cry out for our help and leadership as people continue to be massacred."

In addition to warning that Obama has not done enough to pressure Iran to curtail its nuclear program, Romney has seized on the attack on the consulate in Benghazi, Libya that killed the US ambassador and three other Americans.

Initially, US officials said the September 11 assault was a spontaneous demonstration whipped by outrage over an anti-Muslim film made on US soil.

But they are now describing it as terrorism with possible links to Al-Qaeda, fueling Republican claims that the administration mounted a cover-up to preserve the president's favorable ratings on national security.

Senator Bob Corker signed a letter to national intelligence director James Clapper calling for an independent investigation into Benghazi.

"We've had this bizarre response" to the attacks, Corker told Fox News, saying he believed there might be "gross negligence or incompetence involved."

The comments came as the candidates prepared for what will undoubtedly be one of the most closely watched moments of the acrimonious 2012 race.

Obama heaped pressure on his foe, who is angling for a dramatic turnaround for his ailing campaign, before bunkering down in a resort in Nevada's rocky desert to shake off his mothballed debating skills.

"The media is speculating already on who is going to have the best zingers... who's going to put the most points on the board," Obama told a rally of 11,000 people in a Hispanic suburb of Las Vegas on Sunday.

"Governor Romney, he's a good debater... I'm just okay," Obama said tongue-in-cheek, as he upped the stakes in the classic game of expectation-setting that rival campaigns indulge in before big debates.

"What I am most concerned about is having a serious discussion about what we need to do to keep the country going and restore security for hard working Americans."

Obama currently leads the national race by five points in the latest Gallup daily tracking poll and in most key battlegrounds.

A Washington Post-ABC News poll out Monday gave Obama a slimmer 49-47 percent lead, but, tellingly, 52 percent of likely voters in swing states sided with the president compared to 41 percent for Romney.

And by 55-31 percent, respondents said they thought Obama would fare better in the opening debate than Romney.

While the classic pre-debate spin requires campaigns to play down the chances of their man, gruff New Jersey Governor Chris Christie appeared not to get the memo from the Romney camp.

"Wednesday night is the restart of this campaign and I think you're going to see those numbers start to move right back in the other direction," Christie said.

Romney advisers sounded a more cautious note Monday, as they disputed arguments that his campaign message had lost focus in recent weeks.

"Our message is very clear, which is we cannot afford four more years like the last four years, and we need a real recovery," senior Romney adviser Ed Gillespie said.