Trump has endorsed Mitt Romney - the man he once called the 'dumbest and worst candidate in history'

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Then-President elect Donald Trump and Mitt Romney leave the clubhouse after their meeting at Trump International Golf Club, November 19, 2016 in Bedminster Township, New Jersey.

• President Donald Trump endorsed Mitt Romney in his run for a Senate seat in Utah.
• Trump urged people to vote for Romney in 2012, but the two criticized each other heavily during the 2016 election campaign.
• The history of their relationship can be seen from Trump's many tweets about Romney over the years.

President Donald Trump endorsed Mitt Romney in his run for a Senate seat in Utah on Monday night.

Romney announced last week he would seek to replace retiring Senator Orrin Hatch at the mid-term elections this November. 

Trump responded to the news by endorsing the former presidential candidate on Twitter. He said "he will make a great Senator" and has "my full support and endorsement!"

But the two men have a tumultuous history, going back to Romney's 2012 presidential bid, through to Romney being one of Trump's most vocal critics during the 2016 election.

Warm support in 2012

It started initially with Trump endorsing Romney's campaign, while subtly indicating their was popular support for his own presidential run.

Trump openly encouraged voters to get behind Romney on Twitter and also recorded Romney robo-calls.

Trump's support of Romney seemed unconditional.

Turning sour after Romeny lost

But immediately after the Romney's election loss, Trump seemed to blame the campaign while maintaining a polite level of respect for the candidate.

By the following day, Trump seemed to feel he had been key to Romney's win in six states and the campaign's failure to use him more was the cause of the loss.

In early 2013, Trump felt Romney lost because he didn't connect with voters.

Eighteen months later, as rumours began to swirl about who the 2016 Republican presidential candidates would be, Trump began hitting out at Romney.

Their relationship deteriorated in 2015.

Brickbats over Trump campaign

In September, Romney said he didn't believe Trump would become the Republican nominee because the party usually chose a conservative candidate who "gives people confidence that they can guide the ship of state in troubled waters."

Romney, who had considered a 2016 run before ruling it out in early 2015, doubled down on his critique of Trump's "hurtful comments" on a range of topics, including women and Hispanics, just a few weeks later.

"I think he'll have some challenges if he proceeds to the next stage," Romney said at the time.

The situation deteriorated even further between the two in early 2016.

In February, Romney said the reason Trump hadn't released his tax returns must be because they contain some sort of "bombshell."

"Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying the kind of taxes we would expect him to pay," Romney told Fox News.

Outright insults

Trump responded the next day. At a GOP debate he said Romney ran a "terrible campaign, he was a terrible candidate." He also lashed out on Twitter. 

This time, Romney responded by invoking William Shakespeare's "Hamlet."

On March 3, Romney called Trump a "phony," "fraud," "con man," and "fake."

"Let me put it plainly," Romney said in a speech that day. "If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished."

He also tried to distance himself from his one-time endorsement from Trump, though Trump then claimed Romney had "begged" for his endorsement.

And Trump responded in kind.

'Unfit for office'

The war of words continued the next day with Romney saying Trump "would be terribly unfit for office."

"I don't think he has the temperament to be president," Romney said.

At a rally later that day Trump called Romney a "spoiled brat" and a "nasty guy."

"I don't like him," Trump said. "He thinks he's hot stuff. And I hate people that think they're hot stuff, and they're nothing."

There were so many tweets from Trump flying around Romney took part in the Mean Tweets segment on "Jimmy Kimmel Live" to poke fun at the situation.

Over the next few months increasing numbers of Republican candidates pulled and Trump's path to the party nomination became clearer.

But on June 10, Romney said Trump's election would cause "trickle-down racism, trickle-down bigotry, trickle-down misogyny."

The response from Trump was swift.

Once Trump secured the Republican nomination, little was said publicly between the two for months.

Sharp turnaround after Trump won

But it wasn't until after the 2016 election that their relationship obviously changed.

Romney quickly sent his "best wishes" to President-elect Trump and the two met for dinner at the end of November. Afterwards, Romney said he thought Trump "can lead us to that better future."

The two had several face-to-face meetings in November, and it became clear that Romney was being considered for the Secretary of State. At the time, Trump adviser Kellyanne Conway said supporters would feel betrayed by that decision and, in the end, Rex Tillerson was chosen instead.

Romney responded by saying he had "very high hopes that the new administration will lead the nation to greater strength, prosperity and peace."

In 2017, there was only one tweet between the two.

And 2018 has seen the relationship come full circle with Trump's endorsement of Romney's Senate bid.