President Barack Obama won a major victory this week when Maryland's Barbara Mikulski, after much deliberation, became the 34th U.S. senator to announce her support for the nuclear accord with Iran. Senator Mikulski's pledge to back the president's veto of an expected Republican-sponsored resolution that would kill the agreement means that whatever happens next, the landmark accord negotiated by the administration and its diplomatic partners over the last two years is virtually a done deal. Hers was a laudable example of statesmanship under tremendous pressure. Maryland's junior senator, Ben Cardin, made a similarly exhaustive effort to vet the agreement and announced today that he will oppose it. We understand his reservations. He is right that there is risk either in moving forward with the deal or in opposing it, but we agree with Senator Mikulski that the deal represents the best chance of restraining Iran's nuclear ambitions in the forseeable future and avoiding armed conflict.