Clinton on Monday standing by her threat made last month to "obliterate" the Islamic republic should it use nuclear weapons on Israel, whose US backers form one important constituency in the Democratic nominating race.
Obama accused the New York senator of taking a page out of President George W. Bush's assertive foreign policy, but Clinton herself insists she is not being bellicose, but rather trying to deter Iran from taking a step too far.
Vali Nasr, an Iranian-American academic and member of the Council on Foreign Relations, said there was an element of campaign posturing in Clinton's threat — but he also saw the outlines of a new US approach to Iran.
"Its broader significance is that it shifts from the current debate of how to stop Iran from going nuclear, if Bush's current policies are not working," he told AFP. "She already has outlined a sort of deterrence doctrine."
Clinton's campaign chief strategist Geoff Garin said the debate was about how to stop Iran acquiring nuclear arms in the first place, and that her policy of engagement with Tehran was in marked contrast to Bush's.
Iran's leadership must have no doubt that under a Clinton presidency, their country would face "massive retaliation on our part," Garin said on a media conference call.
"She believes that is the best way to deter an attack," he said.
"More broadly in terms of Iran, Senator Clinton has talked about the need to move to a different kind of more constructive level of diplomatic engagement," Garin added.
"She has a much more nuanced policy that includes a much greater use of diplomatic engagement of Iran than has occurred under President Bush."
Obama too wants to bring Iran in from the cold, but goes much further than Clinton in advocating direct talks at a leaders' level.
The Illinois senator denied his own position left any doubt about whether the United States would come to Israel's aid in the event of an Iranian attack.
"An attack on Israel, one of our most important allies in the world, would be considered as an attack on the United States," he said on CNN Monday.
"Using the word obliterate, however, is the kind of language that we've seen George Bush use over the last seven years," he said, accusing Clinton of "stirring up international incidents … right before an election."
The foreign policy exchanges have sharpened as the two Democrats prepare to renew battle in their epic nominating contest on Tuesday, with primaries in Indiana and North Carolina.
In an April 22 interview with ABC, Clinton was asked what she would do as president if Iran were to launch a nuclear strike on Israel.
"I want the Iranians to know that if I'm the president, we will attack Iran," she replied.
"In the next 10 years, during which they might foolishly consider launching an attack on Israel, we would be able to totally obliterate them," she added.
On Sunday, Clinton said she had no regrets about the remark, which triggered an Iranian complaint at the United Nations and a comment by junior British foreign minister Lord Mark Malloch-Brown that her position was "not prudent."
"I sure want to make it abundantly clear to them that they would face a tremendous cost if they did such a thing," the former first lady said.
But Steve Clemons, executive vice president of the left-leaning New America Foundation, said Clinton's tone on Iran was worrying even if it does form part of her campaign narrative against Obama.
"You can pretend it is theater, but this is real-world stuff," he said, recalling Republican contender John McCain's jokey rendition of "bomb, bomb, bomb Iran" to a Beach Boys tune last year.
Underlining that Iran takes such commentary deadly seriously, Clemons warned the White House runners against saying anything that could be "deeply, deeply destructive to American interests."
