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Mitt Romney took his criticism of Donald Trump a step further Friday, saying he wouldn't vote for the man if he becomes the Republican presidential nominee.

Romney appeared on Fox Business the day after delivering a scathing attack on Trump at the University of Utah, an unprecedented move for the past presidential nominee to attack the current front-runner. After Romney's speech, Sen. John McCain, the 2008 GOP nominee, joined in criticizing Trump.

In his address Thursday, Romney didn't address how he would handle a Trump nomination if his efforts to block the celebrity billionaire failed, but he answered that question Friday.

"I'll either vote for a conservative who runs or I'll write in the name of a conservative," Romney said in an interview with Neil Cavuto. "I cannot, in good conscience, vote for a person who has been as degrading, disruptive and unhinged as I've seen Donald Trump be."

Trump endorsed Romney in 2012 and used that in an attempt to undercut the former Massachusetts governor's jabs. He said Romney "begged" for his support and if he wanted, he could have forced him to his knees.

In his Thursday speech, sponsored by the U.'s Hinckley Institute of Politics, Romney suggested that voters should back whichever candidate had the best chance to beat Trump in each state, which would lead to a contested GOP convention unlike anything seen in generations. If that happens, Romney said he wouldn't seek the nomination.

"I'm going to support whoever our nominee is," he said Friday, presumably as long as it isn't Trump. "If I wanted to advance Mitt Romney, I would have filed as a person seeking delegates in the various states."