Milwaukee, Wisconsin - Eight Republican presidential candidates sparred Wednesday over immigration, the economy, and abortion in the first debate of the 2024 US election cycle – but the attention was still on Donald Trump even as he boycotted the showdown.

Trump's snubbing of the two-hour Milwaukee event deprived a chasing pack of rivals, whom he leads by massive margins in polls, of the opportunity to direct shots at him live on stage.

Instead, he gave a recorded interview with former Fox News star Tucker Carlson that was posted online minutes before the debate got underway.

But Trump loomed over the debate, with his multiple prosecutions the subject of questions from the Fox News hosts moderating the event.

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Candidates were asked to signal if they would support Trump as the party's nominee even if he is convicted in one of the criminal cases he is facing.

Every candidate raised their hand except Arkansas governor Asa Hutchinson and former New Jersey governor Chris Christie, who waved his finger.

"Here's the bottom line. Someone's got to stop normalizing this conduct, OK?" Christie said, drawing loud boos from the audience.

"Whether or not you believe that the criminal charges are right or wrong, the conduct is beneath the office of President of the United States," he added.

Hutchinson earned more jeers when he said: "Obviously, I'm not going to support somebody who's been convicted of a serious felony."

The debate moderators opened with a question on hit song Rich Men North of Richmond, prompting Trump's closest rival, Florida Governor Ron DeSantis to warn that the country is "in decline."

"This decline is not inevitable. It's a choice," said DeSantis, who has faded in polls after previously being touted as a serious rival to Trump.

South Carolina senator Tim Scott suggested falsely that Biden had wrecked an economy that was in record shape before the last election, when in fact it was on its knees during the Covid-19 pandemic.

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DeSantis also talked about his record on keeping Florida open during the health crisis, earning a cheer when he discussed how he would have fired government scientist Anthony Fauci.

With a seismic shift needed to dethrone Trump, the debate offered a showcase for candidates angling to be Trump's running mate.

Vivek Ramaswamy spoke often and jumped into clashes as he sought to make an impact – as well as perhaps make a case for a role in any future Trump administration.

Trump's vice president, Mike Pence, said he was the "best prepared" candidate for office, but was booed during an exchange with Ramaswamy for calling the political newcomer a "rookie."

Ramaswamy described himself as "the only person on stage who is not bought and paid for," earning a slapdown from Christie, who complained he'd "had enough already tonight of a guy who sounds like ChatGPT."

The candidates equivocated on climate change – except for Ramaswamy, who called it "a hoax" – railed against street crime, and supported curbs on abortion access – an issue that polarizes America, with Pence rebuking former UN ambassador Nikki Haley over her call for "consensus" on the issue.

DeSantis was asked whether Pence was right to certify the results of the 2020 election, which Trump claims falsely was stolen, and said that "Mike did his duty – I have no beef with him."

But he added: "This election is not about January 6th of 2021. It's about January 20th of 2025, when the next president is going to take office."

DeSantis stayed out of the fray for much of the evening, but was at his most animated when advocating for the use of lethal force to curb illegal immigration.

"When these drug pushers are bringing fentanyl across the border, that's going to be the last thing they do," he said. "We're going to leave them stone-cold dead."

The Biden campaign bought expensive ad slots on Fox News and its website before the debate, while the president said he would watch as much of the event "as I can."