Partisanship has become the pre-eminent scapegoat for American political failure, an all-purpose explanation for Washington’s inability to act. Yet in rare circumstances, it can actually guarantee action. So, notwithstanding an acrimonious debate, polarization makes the United States’ acceptance of the Iran nuclear deal very likely. That is because the mechanism for congressional consideration, agreed on by Republican leaders and the White House , reverses the typical legislative imperative. Instead of requiring an extraordinary majority to act, it requires one to stop action — which means partisanship is all President Obama needs to approve the deal. The Republican-controlled House and Senate can pass a resolution next month disapproving the deal, as lawmakers in both parties expect they will. Mr. Obama can then veto that resolution, as he has promised to do. To override that veto and block the deal, Republican leaders would need two-thirds majorities in each chamber. […]