As part of an opening foray into foreign policy, Ryan criticized the Obama administration's failure to head off Iran's nuclear program.

"They are four years closer to a nuclear weapon," Ryan said. Biden, who prides himself on his foreign policy experience in the Senate and in his current office, defended the program of international economic sanctions the United States has helped put in place against Iran.

"The ayatollah sees his economy being crippled," he said, arguing that Iran was far from even achieving the level of enrichment needed to make a nuclear bomb. Iran has denied that that is its intention, but most countries do not believe its ambitions are to generate peaceful nuclear power only.

Biden suggested that Ryan — and by extension Romney — should acknowledge if they intend to go to war. He said, 'War should always be a last resort."

But Ryan, better known as a domestic policy expert, appeared eager to take on the Obama-Biden foreign policy record. He accused the administration of inconsistency in its relationship with allies, and for sending the wrong signals to antagonists.

In a colorful aside, Biden called Ryan's contentions "a bunch of stuff," a euphemism for something else.

"Facts matter," Biden said.