On Thursday, Romney said he was honored to receive the endorsement, but hoped even more to win the support of Nevada voters. The state holds presidential caucuses Saturday.
Trump, for his part, called Gingrich "a wonderful person" but said he had decided to endorse Romney two weeks ago.
"He's a friend of mine, I like him a lot. I respect him a lot. But this is the way I went," Trump said.
Trump said China policy was a key factor in his decision to back Romney. Trump has often accused China of manipulating its currency and "cheating" the U.S. — a theme Romney has echoed on the campaign trail.
At a later event, Romney criticized the Obama administration on a different point of foreign policy — the possibility of releasing several inmates from the U.S. prison at Guantanamo Bay, Cuba, as an incentive for the Taliban to join peace talks.
"A sign of good faith? Are you kidding me? A sign of good faith to our enemy?" Romney said.
Trump, a multimillionaire and host of a reality show where he famously proclaims "You're fired," said he wasn't worried that his endorsement would hurt Romney, another wealthy businessman whose opponents have criticized as out-of touch.
"I really think he does connect, and he's starting to connect really well," Trump said.
Democrats, for their part, signaled they believe Trump's endorsement of Romney could work against him. The campaign sent out video of the endorsement event on the president's Twitter feed, with no added comment.
Rival candidate Rick Santorum said of the Trump endorsement, with no small amount of sarcasm: "I can't imagine Donald Trump being hypocritical. That would be totally out of character for Donald Trump."
Santorum added: "You know, Donald Trump does things for one reason and we all know what that reason is. And I think people the people of Nevada are smart enough to figure out who they're going to vote for. It's not going to be based on what Donald Trump recommends."
Associated Press writers Kasie Hunt, Shannon McCaffrey and Philip Elliott contributed to this report.
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