US President Joe Biden will give a key speech in Arizona Thursday on protecting democratic institutions, in a fresh attack on Donald Trump as the rivals ramp up their 2024 election battle.

The White House said Democrat Biden's speech will also "honor the legacy" of late Arizona senator John McCain, the moderate Republican stalwart who shared his antipathy for Trump.

With polls showing them neck-and-neck, the 80-year-old Biden has recently stepped up attacks on Republican frontrunner Trump for trying to "destroy" democracy.

Populist tycoon Trump faces criminal charges for trying to subvert the 2020 election, which his supporters tried to overturn by attacking the Capitol on January 6, 2021.

Biden's speech in Tempe, Arizona "will focus on the importance of America's institutions in preserving our democracy and the need for constant loyalty to the US Constitution," a White House official said.

"The president will honor his friend and war hero, the late senator John McCain, whose intolerance for the abuse of power and faith in America sets a powerful example to live by," the official added.

Invoking the memory of McCain, a Vietnam war hero and respected US political figure, is a clear bid by Biden to draw a contrast with the hard-right Trump.

- 'Destroy American democracy' -


McCain loathed Trump, withdrawing his support for him in the 2016 presidential election and saying before his death from brain cancer in 2018 that he didn't want him at his funeral.

The feeling was mutual, with Trump mocking his war hero status in 2015.

Biden, on the other hand, delivered a eulogy at McCain's funeral, saying that he "loved" him and that their friendship transcended political differences.

The US election is still more than a year away but this week has seen Biden and Trump locked in their first major head-to-head skirmishes of the campaign.

The pair made dueling visits to car workers in the battleground state of Michigan, with Biden standing on a picket line on Tuesday and Trump addressing employees on Wednesday.

Biden then set off on a series of campaign events, which he used to train his fire on Trump on the issue of democracy again.

"Donald Trump and the MAGA Republicans are determined to destroy American democracy because they want to break down institutional structures," he told a reception in California late Wednesday.

Biden's speech also comes the day after Republican candidates debated in California without Trump, who has said he is so far ahead of them in the polls that he doesn't need to turn up.