Doug Mills/The New York Times

President Obama challenged his Republican critics to make a case to the American people for a military strike against Iran’s nuclear facilities if they really believe that that is the right course to follow, throwing down an election-year challenge to the men who are vying to succeed him and who say that his Iran policy has been too weak.

“This is not a game,” Mr. Obama said during a news conference at the White House timed to coincide with Super Tuesday voting in the Republican primaries in a number of crucial states. Mr. Obama gave a staunch defense of his administration’s actions to rein in Iran’s nuclear ambitions and said tough sanctions put in place by the United States and Europe were starting to work and were part of the reason Iran had returned to the negotiation table.

“The one thing we have not done is we have not launched a war,” Mr. Obama said. “If some of these folks think we should launch a war, let them say so, and explain to the American people.”

Mr. Obama’s comments followed speeches Tuesday morning at the conference of the American Israel Public Affairs Committee, during which Rick Santorum derided the negotiations with Iran as “another appeasement, another delay, another opportunity for them to go forward while we talk,” and Mitt Romney, before the same group, said, “Hope is not a foreign policy.” Mr. Romney added, “The only thing respected by thugs and tyrants is our resolve, backed by our power and our readiness to use it.”

Mr. Obama hit back hard. “There’s no doubt that those who are suggesting, or proposing, or beating the drums of war, should explain clearly to the American people what the costs and benefits would be,” he said, reflecting a belief within the administration and the Obama campaign that Americans, after a decade of war in Afghanistan, Iraq and Libya, are fed up with conflict if it can be avoided through diplomacy and economic pressure.

Beyond that, Mr. Obama said that administration and intelligence officials — in the United States and in Israel — believe that Iran has not acquired a nuclear weapon yet, and that there is still time for sanctions to force the Iranian regime to give up any weapons program it might have. “At this stage it is my belief we have a window of opportunity,” Mr. Obama said.

“And so this notion that somehow we have a choice to make in the next week or two weeks or month or two months is not borne out by the facts,” the president said.

He repeated a pledge he made on Sunday when he addressed the Aipac pro-Israel conference, that he would not “countenance Iran getting a nuclear weapon, adding, “My policy is not containment.”