Both Mr. Obama and Mr. Romney say a nuclear Iran is “unacceptable,” and both hold out the prospect of military strikes to stop Iran as a last resort. But they use different terminology to describe the threshold that would presumably trigger preemptive US military action.

Obama says he would not accept Iran possessing a nuclear weapon, while Romney says he would not accept Iran reaching “nuclear weapons capability” – a lower threshold that suggests a fuzzier point at which military action against Iran would be undertaken.

To halt Iran’s nuclear march, Romney says he would first impose “crippling” sanctions – the same word Obama administration officials use to describe the sanctions they have already put in place. Romney also says he would order aircraft carriers to maintain a regular presence in both the Persian Gulf and the eastern Mediterranean as a means of convincing Iran that the US is serious about a military option if it fails to halt its nuclear program.

Both with and without the United Nations, Obama has imposed on Iran some of the severest economic sanctions ever leveled against a country.

The Obama administration also launched covert operations against Iran’s nuclear facilities, part of a covert war, presumably in cooperation with Israel, that have included cyberworms attacking uranium enrichment operations, explosions at nuclear facilities, and assassinations of Iranian nuclear scientists – though it is not clear that the US is involved in all aspects of this war.

Meanwhile, Obama and administration officials have focused on reassuring Israel that there is still time to see if sanctions and diplomacy can work before military action is necessary. Romney says he would respect Israel's right to take preemptive action against Iran nuclear sites if it decides to launch air strikes.