"There's plenty of evidence that Trump is a con man, a fake. Trump has changed his positions not just over the years, but over the course of the campaign," he said.
"We will only really know if he's a real deal or a phony if he releases his tax returns and the tape of his interview 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," he said.
In a rare speech, Romney gave a clarion call to his party men not to let Trump become the presidential nominee as he is the "wrong person" to lead the party and the country.
"Here's what I know: Donald Trump is a phony, a fraud," Romney said. Romney, who was defeated by President Barack Obama in his 2012 re-election, joins a growing chorus of anxious Republican leaders who appeared to have ganged up against Trump in a last ditch effort to prevent him from becoming the nominee for the November presidential poll.
Trump so far has won 10 of the 16 States where Republican primaries and caucus have been held in the last one month and has a substantial lead in terms of number of delegates as against the three other major Republican candidates -- TedCruz, Marco Rubio and John Kasich.
Claiming that there is a "bombshell" to hide in the Trump's tax returns, which he has not released it so far, Romney said Trump's nomination would only help Hillary Clinton become the next president of the United States.
Romney alleged "dishonesty" is Trump's hallmark.
"He claimed that he had spoken clearly and boldly against going into Iraq. Wrong. He spoke in favour 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 said.
"He's not of the temperament of the kind of stable, thoughtful person we need as a leader. His imagination must not be married to real power. The president of the United States has long been the leader of the free world," he said.
Think of Trump's personal qualities, Romney urged his party men.
"The bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics. You know, we have long referred to him as 'The Donald'," he said.
"He's the only person in the entire country to whom we have added an article before his name, and it was not because he had attributes we admired," he said.
Romney was soon joined by his senior party leader Senator John McCain who endorsed his speech that slammed Trump.
"I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech on Thursday. I would also echo the many concerns about Trump's uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defence and foreign policy leaders," McCain said in a statement.
In his speech, which was telecast live by all major television channels, Romney said Trump lacks the temperament and judgement to be a president.
"And his personal qualities would mean that America would cease to be a shining city on a hill," Romney alleged.
"I understand the anger Americans feel today. 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," he said.
"Donald Trump says he admires Vladimir Putin, while has called George W Bush a liar. That is a twisted example of evil trumping good," he said.
Romney said that the president of the US has long been the leader of the free world.
"The president and yes the nominees of the country's great parties help define America to billions of people. All of them bear the responsibility of being an example for our children and grandchildren," he said.
"We don't have a nominee yet. This thing still has a ways to play out, and so people who are in the party are gonna be speaking their minds while we're selecting a nominee and so everything's fair game on the way to the nomination," Romney said.
Romney challenged Trump to release his tax returns or the transcripts of his meeting with The New York Times editorial board.
"I predict that despite his promise to do so, first made over a year ago, that he will never ever release his tax returns. Never -- not the returns under audit; not even the returns that are no longer being audited. He has too much to hide. Nor will he authorise the release of the tapes that he made with the New York Times," Romney said.
"If I'm right, you'll have all the proof you need to know that Donald Trump is indeed a phony. Attacking me as he surely will won't prove him any less of a phony. It's entirely in his hands to prove me wrong. All he has to do is release his back taxes like he promised he would and let us hear what he said behind closed doors to the New York Times," he said.
Trump disparaged Romney in a series of tweets, including "I am not a Mitt Romney, who doesn't know how to win."
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