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.

Iran’s president says his country will continue talks over its rapidly advancing nuclear program but will not withdraw from its rights because of U.S. threats