Tallahassee Mayor Andrew Gillum upset a crowded field of well-funded candidates in Florida's Democratic primary, a step away from becoming the state's first black governor.

His surprise victory sets up a November showdown with Republican Congressman Ron DeSantis, a favourite of President Donald Trump.

Gillum's victory could further energise black voters in a year when Democrats are already counting on high turnout among minorities to buoy the party in the November election.

Gillum is the party's third black governor nominee this year, with Stacey Abrams in Georgia and Ben Jealous in Maryland. He was endorsed by former presidential candidate Bernie Sanders.

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DeSantis rode Trump's endorsement to victory in the GOP race, underscoring the President's broad appeal in Republican primaries. Trump cheered DeSantis' victory on Twitter, writing: "Ron will be a fantastic Governor. On to November!"

DeSantis was one of several Republicans running in contests in Florida and Arizona who hoped that cosying up to the President would be rewarded by voters.

In Arizona, primary contests were shadowed by the death of Senator John McCain. Though McCain was a towering figure who was elected to the Senate by Arizonans six times, the three Republican candidates running to replace his retiring seat-mate, Senator Jeff Flake, aligned themselves more with the President than the longtime senator.

Arizona Governor Doug Ducey planned to name a replacement to fill McCain's seat after the primary.

- AP

US Senator Bernie Sanders and Democratic governor hopeful Andrew Gillum in Tampa, Florida, earlier this month. Photo / AP file
US Senator Bernie Sanders and Democratic governor hopeful Andrew Gillum in Tampa, Florida, earlier this month. Photo / AP file