Trump was responding to Romney's speech in which he called Trump a fake, a misogynist and dangerous.
In a forceful, top-to-bottom indictment of Trump, delivered as pandemonium sweeps the GOP, Romney urged fellow Republicans to reject the billionaire businessman's candidacy in an election "that will have profound consequences for the Republican Party and more importantly, for the country."
"His promises are as worthless as a degree from Trump University," Romney said in a speech at the University of Utah's Hinckley Institute of Politics. "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."
Romney briefly considered running again before announcing in January that he would pass. His speech in Utah, where he has a home, came at a critical juncture for the Republican Party, with Trump's march to the nomination setting off alarm among GOP leaders who fear that he would lose the general election and tarnish the party's brand for a generation.
"I understand the anger Americans feel today," Romney said. "In the past, our presidents have channeled that anger, and forged it into resolve, into endurance and high purpose, and into the will to defeat the enemies of freedom. Our anger was transformed into energy directed for good. Mr. Trump is directing our anger for less-than-noble purposes."
Romney has not endorsed a candidate in the 2016 race, and his associates said Wednesday that he was unlikely to do so in the immediate future. He has kept in touch with two remaining candidates, Sen. Marco Rubio of Florida and Ohio Gov. John Kasich.
Of the remaining candidates, Romney said, "the only serious policy proposals that deal with the broad range of national challenges we confront come from Ted Cruz, Marco Rubio and John Kasich."
In the speech, Romney also noted: "A person so untrustworthy and dishonest as Hillary Clinton must not become president. Of course, a Trump nomination enables her victory."
The former presidential candidate has emerged in recent days as an aggressive critic of Trump, using Twitter to call on the billionaire to release his tax returns and to denounce Trump's refusal to disavow the endorsement of his campaign by former Ku Klux Klan leader David Duke.
Several of Romney's friends, allies and former donors are involved in efforts to stop Trump, launching and funding super PACs airing ads against the businessman, in Florida, Ohio and elsewhere.
Speaking to students last month at Babson College in Massachusetts, Romney said he shared the feeling of many Americans that Washington has failed them, and he urged national leaders to tackle big problems such as climate change, poverty, education and income inequality.
"We're just mad as hell and won't take it anymore," Romney said of the national electorate. He harshly criticised "the failure of current political leaders to actually tackle major challenges, or to try at least, or to go out with proposals."
In his remarks, Romney criticised Trump's domestic and foreign policies as well as his brash personal qualities, saying that Trump lacked the temperament to serve as president.
"Imagine your children and your grandchildren acting the way he does," Romney said. "Would you welcome that?"
