“Yeah,” he said, “and I laid out what I would do to keep Iran from reaching that red line.” He talked of seven steps, including “crippling sanctions” that, if they had been put in place earlier, would have the effect that “their economy would be on its knees, at this point.” He made no mention of the sanctions Mr. Obama has engineered on Iran, including on its oil sector, depriving the government of about a million barrels a day of oil exports, the country’s most important source of revenue.

Mr. Stephanopoulos gave Mr. Romney another chance. “But your red line going forward is the same?”

“Yes,” Mr. Romney said, going on to describe how it is important that the United States “means what it says.”

“You’ll take any action necessary to prevent that development,” he said, “which is Iran becoming nuclear.”

Mr. Romney’s campaign insisted that he was not changing his position. “As he said this morning, Governor Romney’s red line is Iran having a nuclear weapons capacity,” said Andrea Saul, a spokeswoman. But the imprecise language suggested that Mr. Romney had stumbled over the distinction at the core of the debate over whether a military strike against Iran is justified.

The Obama administration makes the case that the only sensible red line is the acquisition of an actual weapon. After the experience of the Iraq invasion, they note, no president would want to go to war over suspicions about the size of Iran’s nuclear stockpiles of uranium, or the mystery of how far Iran has gone toward engineering a weapon.

In Mr. Obama’s mind, it makes little sense to put war-and-peace decisions on a numerical threshold, rather than having maximum flexibility to find a negotiated settlement. And administration officials insist that if Iran was to race for a bomb, it would be detected in plenty of time to act.

But there is another camp, the one Mr. Romney clearly seemed a member of, at least until he sat down with Mr. Stephanopoulos. An Iran that is just a screwdriver turn away from a bomb will get all the influence that comes with being a nuclear weapon state, this group believes. “Once they get a weapon, or on the verge of getting it, it’s too late,” Mr. Cohen said on Thursday.

This wording is new to Mr. Romney, and one of his advisers said Friday that “you’ve got to give him some slack.” After all, even veterans make the mistake. Defense Secretary Leon E. Panetta said at a large conference last December that the United States would stop Iran from getting nuclear capability. Reminded of administration policy, he soon changed his tune.