Lee, a Tea Party stalwart and former Constitutional lawyer, was Cruz’s top Senate ally in the 2013 effort to defund Obamacare, which resulted in a 16-day government shutdown. The episode turned Cruz into a right-wing rock star, but it also earned him many Republican enemies on Capitol Hill, and until now he hasn’t received the endorsement of a single Senate colleague. Lee will be the first sitting senator to formally back Cruz’s candidacy.

Lee opted to stay on the sidelines for most of the 2016 primary race, which has featured three candidates whom he considers to be his closest friends in the Senate: Cruz, Rand Paul, and Marco Rubio.

Rubio is polling well in Lee’s home state of Utah, and his campaign had hoped to score the senator’s endorsement.