It's time for the United States to hold Iran accountable.

The announcement that a prominent conservative cleric will run for Iran’s presidency next month has transformed the race, potentially unifying opponents to President Hassan Rouhani in a strong challenge to his re-election.

Ebrahim Raisi declared his candidacy on Thursday, according to Tasnim news agency, a day after two other conservatives, former president Mahmoud Ahmadinejad and Tehran Mayor Mohammad Baqer Qalibaf, bowed out. The emergence of a single strong opponent could not only narrow Rouhani’s chance of winning on May 19, but also position Raisi as the front-runner to succeed 77-year-old Ali Khamenei as Supreme Leader, when he dies.

“This is going to be a very serious race with huge consequences for the Iranian electorate,” Sanam Vakil, an associate fellow at Chatham House, said by phone. “The stakes are very high.”

The decision to run represents both opportunities and risks for Raisi and his conservative backers. Beyond the powers of the presidency, victory would increase his chances of succeeding Khamenei, who himself served two terms as president under Ayatollah Ruhollah Khomeini, the leader of Iran’s 1979 Islamic revolution. Failure could take the momentum out of his career and ruin his chance to win Iran’s top role.

“The regime wouldn’t allow someone like Raisi to run without wanting him to win,” said Vakil. “To risk his credentials on the presidency only makes sense if he is going to survive the electoral process, with reputation and credibility intact.”

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