And on Tuesday's "Jimmy Kimmel Live," as part of the show's "Mean Tweets" segment, Romney read one of Trump's scathing tweets, as well as tweets written by Trump supporters.
"I have some tweets, these are real tweets by various parties, including your friend Donald Trump, so if you don't mind I'll hand you each one," Kimmel told Romney, himself a former GOP presidential candidate. "It'll be painless, if you just go through them."
The first tweet Romney read was written by Trump: "Mitt Romney had his chance and blew it," Trump wrote. "Lindsey Graham ran for president, got ZERO, and quit! Why are they now spokesmen against men? Sad!"
Romney shot back, "Got zero? Got zero? The only people I know who got zero are the ones who paid twenty-five grand to be at Trump University." The audience roared with laughter.
Romney read another tweet, from an apparent detractor, who wrote "Donald Trump is trying to pull America back from the brink and freakin' Mitt Romney is playing with matches in the bathroom."
"I'm not touching that, I'll tell you that," Romney told Kimmel.
After reading other tweets, Romney told Kimmel, "Let's make people happy and let's have a candidate we can be proud of."