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.

For a third straight day, the United States on Wednesday issued sanctions targeting Iran as the Trump administration attempts to negotiate a new nuclear arms deal with the Middle Eastern country.

The negotiations are fraught with a history of enmity and a lack of trust, even as both Washington and Tehran seek a narrow deal.