BOCA RATON, Florida — Four years of "failed leadership" by US President Barack Obama has dragged the Middle East to the edge of a major crisis that could dramatically destabilize the region, Senator John McCain warned Monday.

McCain laid down a withering critique of the commander-in-chief's foreign policy just hours before Obama and Republican nominee Mitt Romney face off in Florida in their third and final presidential debate.

"Look at our relations with Russia. Four years and no progress with Iran on nuclear weapons, strained relations with Israel, Al-Qaeda coming back strongly in Iraq, things deteriorating in Afghanistan," McCain fumed.

Monday night's debate, which focuses on international affairs, will be a chance for Romney to tag the president as a weak leader, a label he hopes will stick until Americans head to the polls on November 6.

"The foreign policy debate is about leadership, how the president can answer for our failed leadership throughout the world," McCain said.

He expressed particular anger about the president's inaction on Syria, where a revolt against strongman Bashar al-Assad's regime has left more than 34,000 people dead, according to activists.

McCain, who lost the 2008 election to Obama, warned of arms continuing to pour in to the Syrian regime from Russia, and Iranian agents on the ground in the country.

"And what has this administration done? Absolutely nothing. And just as many of us had predicted, it's beginning to spill over -- into Lebanon, into Jordan, into the other countries in the region, and we are on the verge of a serious crisis."

The influential senator has been among Congress's strongest advocates for broader intervention in Syria, including greater efforts to arm Syrian rebels, a position Romney shares.

With Romney widely seen as besting Obama in the first debate, and Obama seen as making a comeback to win the second with fiery retorts to Romney on issues like Libya, Monday's showdown will be crucial.

McCain sniped about Obama's performance in last week's debate, saying "he wasn't presidential" in his aggressive tone with Romney.

McCain described the president's attacks as "a sign of desperation, but also, not a lot of class."