But Romney dismissed the suggestion that he is positioning himself to enter the ring.
"That's not going to happen," he told CNN's Chief Political Analyst Gloria Borger. "What's going to happen in a contested convention is that people who are running for president and who have delegates are going to be able to battle with one another."
Even so, when pressed on whether he would allow himself to be nominated at an open convention, he sidestepped a complete answer.
"I'm not going to even go there," Romney said. "I have no interest in talking about that."
Despite his effort to bring Trump down, Romney said the billionaire has the best shot at the Republican nomination.
"He's probably most likely to be the Republican nominee at this point. But, um, I think there's a better choice out there," Romney said.
A total of 1,237 delegates are required to win the Republican nomination. Trump currently enjoys a comfortable lead in the race, but a strong showing from Ted Cruz on Super Tuesday, when he won his home state of Texas and two others, and with winner-take-all home-state contests for Florida Sen. Marco Rubio and Ohio Gov. John Kasich looming on March 15, there is still a narrow path for his rivals.
"I'd probably again encourage whoever is doing best in a particular state to get the support there," he said of the other remaining candidates, "and do that state by state. And that would lead to an open convention, where you'd see the delegates selected make the final decision."
Romney said he thought Jeb Bush would make more of an impact in the race.
"I thought Jeb Bush would do better than he -- than he did," he said. "I like Jeb a lot. I thought he would be able to gain a lot of momentum. But that didn't happen."
Romney took issue with Trump's since-reversed proposal
that as president he would have U.S. troops "go tougher than waterboarding." Trump most recently made the comments at Thursday night's Fox News debate.
"The troops then would be guilty of crimes," Romney said. "And (Trump) would be guilty of high crimes and misdemeanors."
When asked about a CNN report
that Trump will raise outside money if he becomes the Republican nominee, Romney said, "it's a form of hypocrisy that I think people will find shocking."
Romney thinks Trump's rhetoric is worse now than in 2012
During his 2012 presidential bid, Romney sought and welcomed an endorsement from Trump -- the same man he on Thursday repeatedly insulted and called a "phony" and a "fraud."
On Friday morning, Romney said he believed the billionaire's current rhetoric was more damaging than his past efforts to stoke the anti-Obama birther movement.
"I think (the birther activities were) very different than calling Mexicans rapists, than saying that Muslims are not going to be allowed into the country as immigrants, that mocking a disabled reporter" or "going after women and saying 'Oh, she asked tough questions because she was in her menstrual cycle," Romney told Borger.
Standing up to Trump for his grandkids
So why did the former Massachusetts governor, now effectively retired from politics, see fit to go public with his attacks on Trump?
"I don't know what impact these things have politically," Romney said, "but I do know that when my grandkids say, 'What did you do to stop Donald Trump?,' I want to be able to say something. I wasn't going to just sit on the sidelines until the very end."