Milwaukee, Wisconsin - Donald Trump returned to the Republican National Convention Tuesday as his defeated rivals for the nomination lined up to heap praise on the GOP's official presidential candidate.

Three of his former foes took the stage separately during prime time to publicly throw their support behind the former president, including one-time UN ambassador Nikki Haley, who told delegates that "a unified Republican Party is essential for saving" the nation.

Trump, again wearing a white bandage on his ear that was injured in Saturday's assassination attempt, waved, shook hands and said "thank you very much" as delegates burst into an ovation.

While he made no formal remarks, the 78-year-old backslapped and chatted with his new running mate J.D. Vance and others, pumping his fist and looking more relaxed than during his emotional entrance the previous night.

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The Republican leader has been widely feted at the four-day convention, where on Thursday he is set to formally accept his party's nomination as challenger to Democratic President Joe Biden in November.

Tuesday was an exercise in presenting a unified front, but the smattering of boos that echoed with cheers across the convention floor when Haley took the stage – with Trump looking on – suggested the party had work to do to gather the base's four corners for the final four months of a bruising presidential campaign.

Haley had antagonized Trump during the primary battle, but on Tuesday she was clear: "Donald Trump has my strong endorsement, period."

Her fellow former Trump challengers – Florida Governor Ron DeSantis and businessman Vivek Ramaswamy – also kissed the ring, with the billionaire telling the 2,400 Republican delegates: "Donald Trump is the president who will actually unite this country, not through empty words, but through action."

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Haley, who this year said the US can't "go through four more years of chaos" under Trump and had refused to endorse him, had not been expected to appear.

But Saturday's shooting at a Pennsylvania campaign rally reshuffled the deck, and Haley joined the speaking schedule along with Ramaswamy and DeSantis.

"Take it from me: I haven't always agreed with president Trump," Haley said, "but we agree more often than we disagree."

Amid calls to lower the temperature of the heated political rhetoric, House Speaker Mike Johnson told the convention Tuesday that "we're not just uniting as Republicans, we're uniting today as Americans in the wake of the assassination attempt on the life of president Trump."