Bangladesh, Nov. 6 -- US President Donald Trump has built much of his political identity around the art of dealmaking. He frequently boasts of achieving what others could not, presenting himself as a leader capable of breaking diplomatic deadlocks and securing extraordinary agreements. Yet an unexpected reversal in his political fortunes may have created an opportunity to pursue the most consequential deal of his presidency: a historic reconciliation with Iran.

The Democrats stunning performance in the US midterm elections, securing control of both chambers of Congress and several key governorships, has changed Washingtons political landscape. The results have also raised questions about the future of Trumps political influence, includin...