Sanctions against Iran’s oil exports that the president signed into law on New Year’s Eve started a fateful clock ticking. In late June, when the campaign is in full swing, Mr. Obama will have to decide whether to take action against countries, including some staunch allies, if they continue to buy Iranian oil through its central bank.

After fierce lobbying by the White House, which opposed this hardening in the sanctions that have been its main tool in pressuring Tehran, Congress agreed to modify the legislation to give Mr. Obama leeway to delay action if he concludes the clampdown would disrupt the oil market. He may also invoke a waiver to exempt any country from sanctions based on national security considerations.

But using either of those escape hatches could open the president to charges that he is weak on Iran, which is viewed by Western powers as determined to achieve a nuclear weapons capability and which has drawn a tough response from Europe as well…

An early test of the administration’s approach will come at the end of February, when the law mandates that it cut off private financial institutions that conduct non-oil transactions with Iran’s central bank, except for the sale of food, medicine and medical devices.