Donald Trump imposed more than 1,000 sanctions on Iran as president, but one of them could prove to be a “poison pill” that derails an effort by his successor to revive the 2015 nuclear deal designed to prevent Iran from building an atomic bomb.

Trump withdrew the U.S. from the deal in 2018 and blacklisted Iran’s Revolutionary Guards, a powerful arm of the Iranian military, as a foreign terrorist organization in 2019. Now negotiations aimed at renewing the nuclear deal are at an impasse over the sanction, with Iran demanding the Biden administration lift the U.S. terrorism designation, according to a current official and three sources familiar with the discussions.

The discussions between Iran and world powers came tantalizingly close to clinching an agreement in late February but became bogged down after Russia raised fresh concerns and as Iranian officials pushed for the lifting of the terrorism designation on the Islamic Revolutionary Guard Corps, or IRGC, the sources said.