If Senator Obama’s presidential campaign collapses, it will be not only because of his shifting explanations in respect of his former pastor, the Reverend Jeremiah Wright, whom he first said he couldn’t disown and then did. It will be not only because of his determination to run to the left of Senator Clinton on the tax issue, promising to raise the payroll tax and at the same time dismissing the idea of a summer suspension of the 18.4 cent a gallon federal gas tax. No, a large part of the explanation will be his decision to run to the left of Senator Clinton on the matter of an American response to Iran.

Mr. Obama’s default came yesterday, under questioning by NBC News’s Timothy Russert on “Meet the Press.” The National Broadcasting Company played a video clip of Senator Clinton on “Good Morning America.” Quoth the senator of New York: “Well, the question was, ‘If Iran were to launch a nuclear attack on Israel, what would our response be?’ And I want the Iranians to know that if I’m the president, we will attack Iran. And I want them to understand that. We would be able to totally obliterate them.”

Mr. Russert asked Mr. Obama for his response. What he got from the senator from Illinois who leads the Democrats in the delegate count was one of the most milquetoast responses ever to pass from the lips of an American presidential candidate in wartime. “It’s not the language that we need right now, and I think it’s language that’s reflective of George Bush,” Mr. Obama said. Quoth he: “It is important that we use language that sends a signal to the world community that we’re shifting from the sort of cowboy diplomacy, or lack of diplomacy, that we’ve seen out of George Bush. And this kind of language is not helpful. When Iran is able to go to the United Nations complaining about the statements made and get some sympathy, that’s a sign that we are taking the wrong approach.”

Well, Theodore Roosevelt was the one who spoke of speaking softly and carrying a big stick. Mr. Obama’s announced Middle East policy is starting to look more like someone who speaks loudly about carrying a small stick. The United Nations, where Russia and Communist China sit on the Security Council along with Libya, while the free Chinese democracy on Taiwan is barred from membership, has a lot less credibility than President Bush does. For all Mr. Obama’s dismissive talk about cowboy diplomacy, the Bush administration’s approach to dealing with Iran has been multilateral to a fault, letting the so-called EU-3 of Britain, France, and Germany take the lead on the issue to the point where he has been accused by some Democrats of outsourcing.

Mrs. Clinton has made her own errors in respect of Iran. In February 2007 she took to the floor of the Senate to demand that Mr. Bush get permission from Congress before launching any attack on Iran. In October 2007, she issued a statement that said, “I’ve been concerned for a long time over George Bush’s saber rattling and belligerence toward Iran.” It’s reasonable to wonder whether her current position reflects core beliefs or just political calculation, but whatever the motivation, it sounds good to us.

We don’t discount entirely the possibility that, if Mrs. Clinton could move from her old position to her current “totally obliterate them” posture, there is hope too for Mr. Obama. But it is late in the game. This is the context in which the Sunday Times of London is reporting that the American military is drawing up plans for a “surgical strike” against an Iranian training camp for those killing Americans in Iraq. Mrs. Clinton has been talking not of a “surgical strike” on Iran but of the American ability to “totally obliterate them.” Mr. Obama, on the other hand, seems more comfortable running to the United Nations and complaining about President Bush’s “cowboy diplomacy, or lack of diplomacy.” It will be interesting to see which of those two candidates the undecided Democratic superdelegates who will decide the nomination think has a better chance of defeating Senator McCain in November.