By design or accident, it is increasingly clear that the centerpiece of President Barack Obama’s second-term foreign policy is a nuclear agreement with Iran. Whether Obama can succeed, however, now depends on Congress staying out of the negotiations.

Over the last few weeks, 16 Democratic senators have supported a bill that would impose new sanctions on Iran over its nuclear program. They have defied the White House’s intense campaign to block Congress from adding new conditions to any deal.

In this way, Obama is the victim of an increasingly craven Washington — where members of his own party are abandoning him out of political expedience. At the same time, the White House is also a victim of its sometimes erratic responses to events in the Middle East.

For the last six years, the president has repeatedly declared that he does not want the United States entangled in another conflict in the Middle East. As a result, allies and enemies at home and abroad, from members of Congress to Israeli and Iranian hawks, question his commitment to use force against Iran if negotiations fail.

Experts warn that the stakes are enormous. Political opportunism, maximalist positions and mixed messages could take on a life of their own, scuttle the talks and inadvertently spark military action.