Mitt Romney’s adviser, Eliot Cohen, said on Thursday: Mr. Obama had made a big mistake declaring only that he would stop Iran from “acquiring a nuclear weapon,” Mr. Romney, “would not be content with an Iran one screwdriver’s turn away from a nuclear weapon.”

Mr. Romney is determined to stop Iran from obtaining a nuclear “capability” — the combination of nuclear fuel, the technology to fashion it into a weapon and a delivery device — that would enable it to build a weapon in a matter of weeks or months, Mr. Cohen said.