Romney, the 2012 Republican nominee for president and former governor of Massachusetts, has been one of President Donald Trump’s most prominent critics, referring to then-candidate Trump in a March 2016 speech as “a phoney, a fraud.” As a senator, Romney could prove an irritant for the White House, like Arizona Republican John McCain who has been a prominent voice — and sometimes vote — against some of the president’s policies and actions.

Tensions have thawed in recent months between the current and former Republican standard bearers. Trump backed Romney in his bid to become a senator, while Romney, who thanked the president for his support on his own Twitter feed, told the Associated Press he wasn’t “going to be a daily commentator on everything the president says or does.”

Romney announced in February he would run for the seat of retiring Republican Senator Orrin Hatch, who decided not to seek an eighth term despite Trump’s entreaties.