"In terms of my own plans, I anticipate supporting one of the three, Ted Cruz, Marco Rubio or John Kasich as our nominee," he said on Fox News Sunday. "If they don't become the nominee, then I'm probably going to go to the voting booth and find someone else running as a conservative, or perhaps just write in the name of someone who I believe should become the president of the United States who I could be proud of."
Romney argued that "the great majority of mainstream Republicans from across the country don't want to see Donald Trump as president of the United States either."
Exit polls to date show that only about 49 percent of Republican voters would be satisfied with a Trump nomination, although Rubio and Cruz receive only slightly more support.
But Romney, who said he plans to endorse one of Trump's rivals before the Republican convention this summer, ducked the question of whether he helped legitimize the businessman by accepting his endorsement in the 2012 race.
"A lot of people endorsed me who I wouldn't endorse for president," he said. "Donald Trump just happens to be one of those who endorsed me I do not want to see as president of the United States. And there's a long list of those who are endorsers. Sixty-one million people voted for me. I don't think all 61 million people ought to be president of the United States."
Pressed by Fox's Chris Wallace to name what had changed since 2012, by which time Trump had already seen numerous business failures and repeatedly claimed that President Barack Obama wasn't born in the United States, Romney rattled off a litany of moments from the campaign trail.
"Let's talk about what didn't happen back then and what has happened during this campaign: calling George W. Bush a liar, cozying up to Vladimir Putin and saying he's a strong leader and popular with his people, equivocating on the Ku Klux Klan, talking about all Muslims as if they're one monolithic group," Romney said, calling Trump's policy positions "totally inappropriate."
Romney also pushed back against the idea that his concerns reflected an attempt by the Republican establishment to wrest back control of their party, instead touting the outsider cred of two of Trump's rivals.
"Well, you can't control Ted Cruz, for instance -- no one has suggested you could do that. And Marco Rubio -- everyone tried to stop Marco Rubio from going against a sitting Republican governor in Florida. He did it anyway and won," he said. "'Establishment' suggests that there must be some Wizard of Oz somewhere pulling the strings. That's not the way it works. There are individuals like myself. I sat there and watched Donald Trump. I said, 'Look, someone's got to say something.'"
Romney reiterated that he has no plans to seek the presidency himself, but refused to rule out the possibility that he'd run if drafted by the party.
"No Republican should say that. That makes no sense, for someone to say if they were drafted by their country that they'd say no," he said. "What I can tell you is I'm not running for president. I'm not going to run for president. I'm going to support one of these four people to be our nominee."