Republican John McCain has accused his potential White House rival Barack Obama of a "reckless" misreading of Iran's threat, but the Democrat insisted it was time to turn the page on failed diplomacy.
The two senators pursued the opening shots of what will be a major foreign policy battle if they clinch their parties' nominations and go head to head in November's election.
McCain, addressing the National Restaurant Association in Obama's hometown of Chicago, said his probable adversary was guilty of "inexperience and reckless judgment" for advocating engagement with nations such as Iran.
"Those are very serious deficiencies for an American president to possess," the Arizona senator said, reheating a simmering row after President George W Bush last week accused Democrats of wanting to appease terrorists.
McCain highlighted Obama's remarks Sunday, when the Illinois senator had said Iran, Cuba and Venezuela were "tiny compared to the Soviet Union," but that Washington had never shied away from engaging Moscow during the Cold War.
"They might not be a superpower, but the threat the government of Iran poses is anything but 'tiny'," McCain said.
"Should Iran acquire nuclear weapons, that danger would become very dire indeed."
However, speaking in Montana yesterday as he campaigned to eliminate Hillary Clinton from the Democratic race, Obama insisted: "I understand the threat of Iran.
"What I know is that the Soviet Union had the ability to destroy the world several times over, had satellites spanning the globe, had huge masses of conventional military power, all directed at destroying us.
"So I've made it clear, for years, that the threat from Iran is grave but what I've said is that we should not just talk to our friends, we should be willing to engage our enemies as well.