Reaching a deal with Iran will test the harder-line wing of Mr. Trump’s supporters in the Republican Party and whether they will fall in line with what would be a departure from their longstanding demands that Iran dismantle its nuclear program. More than 200 congressional Republicans urged him in a letter this week to stand firm with Iran.

President Trump indicated the U.S. was close to a deal in which Iran agrees to forgo nuclear weapons, sparking a selloff in oil prices. "You probably read today the story about Iran has sort of agreed to the terms,