Mitt Romney had a wry response to Donald TrumpDonald TrumpBush ethics lawyer: Trump should strip Flynn of military title Dems might begin again with Kamala Harris and California Trump's tax plan builds GOP's path to 2018, 2020, and beyond MORE's endorsement of his presidential campaign.
"There are some things you just can't imagine in your life — this is one of them," he said.
"We really have an opportunity to do something great for the country," Trump said. "It's my honor — real honor — and privilege to endorse Mitt Romney."
Trump, who had pondered his own GOP bid, ended his remarks with: "Go out and get him."
The event lasted less than five minutes and the two men, who were accompanied by Ann Romney on stage, did not take questions from the media.
Trump's formal announcement was preempted by attacks from Democrats and Romney's Republican rivals alike.
Ron Paul didn't seem impressed with Trump's endorsement.
"Please explain to us why anyone would care," stated a release sent out by his campaign.
Rep. Debbie Wasserman Schultz (D-Fla.), the chairwoman of the Democratic National Committee, tweeted: "Next up: Who is Snooki going to endorse?"
And the DNC sent out a release noting Trump endorsed Romney because "they both like firing people."
It's an attack likely to be seen again — Democrats have repeatedly criticized Romney for his comments in New Hampshire that he "likes to be able to fire people." And Trump is known for his closing line on his NBC show, "The Apprentice": "You're fired."
White House spokesman Jay Carney was asked about the endorsement at his briefing. His response: "I'm not going to comb over that question," he said, referring to Trump's hair.
The event in Las Vegas capped early confusion about the real estate mogul's plans: early reports indicated he might endorse Newt Gingrich; others said it would be Romney.
According to The Associated Press, Gingrich told his campaign advisers on Wednesday that he expected to get Trump's endorsement.
Trump started the buzz when he used his Twitter account Wednesday to note he had a "major" announcement about the presidential race.
He had met one-on-one with a number of GOP presidential candidates in 2011, after he announced he wouldn't be a candidate himself. The businessman had often led in polling on the race. In discussing whom he might endorse, Trump said that he had a "good, good sight on everybody."
Romney and Trump did not appear to have had the best of relationships earlier in the GOP race. In early December, Romney was one of a number of candidates who declined to attend a debate that Trump was scheduled to moderate. That debate was eventually canceled.
Earlier last year Trump had also criticized Romney, saying that he was a "small-business guy."
"I’m much bigger than this man and have a much, much bigger net worth. I mean my net worth is many, many, many times Mitt Romney," Trump said.
— This story was updated at 4:13 p.m.