And then he got colorful.
The former Republican presidential nominee blistered Trump for nearly 20 minutes, calling the GOP front-runner a "phony" and a "fraud."
These were the most pointed lines.
"But you say, 'Wait wait wait,' isn't he a huge business success? Doesn't he know what he's talking about? No he isn't. And know he doesn't. His bankruptcies have crushed small businesses and the men and women who worked for them. He inherited his business, he didn't create it. And what ever happened to Trump Airlines? How about Trump University? And then there's Trump Magazine and Trump Vodka and Trump Steaks and Trump Mortgage. A business genius he is not."
On Trump's foreign policy credentials:
"Donald Trump tells us that he is very, very smart. I'm afraid that when it comes to foreign policy he is very, very not smart."
On whether Trump has the temperament of a president:
"This is an individual who mocked a disabled reporter, who attributed a reporter's questions to her menstrual cycle, who mocked a brilliant rival who happened to be a woman due to her appearance, who bragged about his marital affairs, and who laces his public speeches with vulgarity."
On how Trump recalls spending the Vietnam War years:
"There is dark irony in his boasts of his sexual exploits during the Vietnam War while at the same time John McCain, whom he has mocked, was imprisoned and tortured."
On Trump's "hallmark" quality:
"Dishonesty is Trump's hallmark: He claimed that he had spoken clearly and boldly against going into Iraq. Wrong. He spoke in favor of invading Iraq. He said he saw thousands of Muslims in New Jersey celebrating 9/11. Wrong. He saw no such thing. He imagined it. He's not of the temperament of the kind of stable, thoughtful person we need as leader. His imagination must not be married to real power."
On his very specific plan to defeat Trump in the primary:
"Given the current delegate selection process, that means that I'd vote for Marco Rubio in Florida, for John Kasich in Ohio, and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state."
On Trump's less appealing "personal qualities":
"Think of Donald Trump's personal qualities, the bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics. We have long referred to him as 'The Donald.' He is the only person in the entire country to whom we have added an article before his name. And it wasn't because he had attributes we admired."
On what Romney thinks is the most ridiculous idea of the campaign season so far:
"What he said on '60 Minutes' about Syria and ISIS has to go down as the most ridiculous and dangerous idea of the campaign season: Let ISIS take out Assad, he said, and then we can pick up the remnants. Think about that: Let the most dangerous terror organization the world has ever known take over a country? This is recklessness in the extreme."
On what would happen if Trump released his tax returns and a tape of his off-the-record conversation with The New York Times:
"I predict that there are more bombshells in his tax returns. I predict that he doesn't give much if anything to the disabled and to our veterans. I predict that he told The New York Times that his immigration talk is just that: talk. ... If I'm right (that he won't release either), you will have all the proof you need to know that Donald Trump is, indeed, a phony."
On the value of a diploma from Trump University:
"Here's what I know. Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University. He's playing the members of the American public for suckers: He gets a free ride to the White House and all we get is a lousy hat."