Atlanta, Georgia - The leaders of two of the largest US states went head-to-head Thursday in a prime-time debate offering contrasting views of how the deeply-polarized nation should be run – and a preview of how the 2028 White House race might look.

Republican Florida Governor Ron DeSantis and Gavin Newsom, his counterpart in Democratic California, have been hammering each other for years over their vastly differing outlooks, but the at times bad-tempered debate marked their first in-person clash.

The pair represent almost one in five Americans – California is the largest US state and Florida is third – and debate host Fox News had billed the showdown as a clash between liberal blue and conservative red America.

The 45-year-old Florida governor, once seen as the most viable alternative to scandal-plagued Donald Trump for the Republican party's presidential nomination, is desperate to revive a flagging White House bid going into election year.

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Newsom (56) isn't running, but speculation about a presidential campaign in 2028 has grown with his appearances on the stump for President Joe Biden.

Urging Americans to choose "freedom over failure," DeSantis said Newsom was "joined at the hip" with Biden and Vice President Kamala Harris, adding sarcastically that they had "done a great job."

"What California represents is the Biden-Harris agenda on steroids," he said. "They would love nothing more than to get four more years to be able to take the California model nationally, that would be disastrous for working people."

But it was a mixed message as he also accused Newsom of running a "shadow campaign" to usurp 81-year-old Biden in the event that declining cognitive health rendered him unable to run.

Newsom has spent much of the election cycle needling DeSantis over gun control, abortion restrictions, and other so-called "culture war" issues such as the teaching of gender and sexuality in classrooms. He accused his opponent, who has been criticized over book bans in Florida libraries, of "using education as a sword for your cultural purge."

He also blasted the Republican doing "nothing meaningful" on gun safety after the 2018 mass shooting that killed 14 students and three staff at Marjory Stoneman Douglas High School in Parkland.

DeSantis fired back at Newsom on immigration, public health restrictions, and crime, arguing that "leftist policies" had damaged Californian cities like San Francisco and sparked an exodus.

"People are leaving California in droves because he has failed to stand up for public safety. They are on an ideological joyride, to let people out of prison early," the Republican said.

He attacked Newsom over gas and sales tax rates in the Golden State while the Democrat countered that poorer Floridians were paying more in tax overall than rich Californians.

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The 90-minute DeSantis vs. Newsom: The Great Red vs. Blue State Debate took place in a suburb of Atlanta, Georgia, once a Republican bastion that has moved leftwards to become a key battleground.

Moderator Sean Hannity, a Trumpist who is open about his Republican sympathies, had vowed to be neutral, but many of the early topics appeared geared to appeal to the network's largely conservative viewership.

DeSantis appeared more aggressive and yet conversely more at ease than he has in his Republican primary debates, where he has been described as subdued and awkward.

Newsom seemed more comfortable defending Biden's economic record in the early exchanges than his own policies at home, celebrating nationwide job growth and investment in research and development.

There has been bad blood between the pair for months over DeSantis flying undocumented migrants to California, and Newsom's fierce criticism of Florida's six-week abortion ban, which the Democrat called the "most extreme" in America.

Newsom, who has suggested that Florida officials could face kidnapping charges for their involvement in a stunt flying migrants from Texas to Sacramento in June, said DeSantis was "nothing but a bully."

DeSantis countered that California was a "sanctuary state" where officials "thumbed their nose at federal immigration law, and this has real consequences."