He accused Mr Trump of bullying, misogyny and dishonesty in a speech in Utah on Thursday.
“Prospects for a safe and prosperous future are greatly diminished” if Mr Trump becomes the nominee, he added.
Mr Trump has meanwhile mocked Mr Romney on Twitter as a “failed candidate” who should not advise on electability.
Many senior Republicans are alarmed at the prospect of Mr Trump securing the nomination for November’s election.
In the latest attack from top figures in the party, several members of the Republican national security community wrote an open letter describing Mr Trump’s “vision of American influence and power in the world” as “wildly inconsistent and unmoored in principle”.