"He will make a great Senator and worthy successor to @OrrinHatch, and has my full support and endorsement!" tweeted Trump (or whoever had his phone) on Monday evening.
Romney and Trump have a hate-hate relationship, notably trading barbs during the 2016 election, with Romney calling Trump a "phony" and a "fraud" to a crowed of nearly 700 at the University of Utah, and Trump calling Romney a disloyal "choke artist" and "failed candidate" who was eager for Trump's endorsement during his unsuccessful 2012 Presidential bid.
"You can see how loyal he is," Trump said. "He was begging for my endorsement. I could've said, 'Mitt, drop to your knees,' and he would've dropped to his knees. He was begging. True. True. He was begging me." -WaPo
After Trump won the election, he trolled Romney with the possibility of the Utah politician becoming Secretary of State - only to give the job to Exxon CEO Rex Tillerson.