Los Angeles, California - President Joe Biden headed to Los Angeles for a star-studded fundraiser bringing millions of dollars for his election fight as Donald Trump visited Michigan to lead a convention of far-right Republicans in his own fundraising bid.

The president was joined by Barack Obama, the previous Democratic commander-in-chief, at an event that included remarks by actors Julia Roberts and George Clooney, as well as singing icon Barbra Streisand.

Despite the Hollywood heavyweights, Biden turned serious when he spoke of his rival Trump and how whoever wins the election will likely have at least two new Supreme Court nominations to make.

"The idea that if he's re-elected, he's going to appoint two more flying flags upside down," Biden said, referring to recent tumult over a conservative sitting justice who was recently confirmed to have had an inverted American flag – a symbol of Trump's false election fraud claims – raised outside his home in 2021.

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Trump – making an unprecedented bid to win back the White House while running as a convicted felon – was also on the campaign trail, boasting in Detroit, Michigan, that his own fundraising is "the highest in the history of politics."

Michigan is a must-win state for Biden in November's electoral mathematics.

Aiming to eat into Biden's key electoral support from African Americans there, Trump visited a Black church in Detroit and told hundreds of voters that "crooked Joe Biden has done nothing for you except talk."

Trump then headed to a starkly different venue: a convention of high-profile hard-right Republicans and supporters of his attempt to overturn the 2020 election.

At the Turning Point USA convention, Trump railed against Biden's climate protection package, mocked Ukrainian President Volodymyr Zelensky as a "salesman," and renewed his incendiary rhetoric about what he branded the "Biden migrant invasion," saying he will stop it with the biggest deportation operation in American history.

In a characteristically rambling 80-minute speech – frequently interrupted by loud cheering – Trump claimed that help for migrants leaves US war veterans "lying in the streets" and veered into everything from extended complaints about modern showers to repeating his lie that his 2020 election loss was "rigged and stolen."

"We have a rigged country. We have rigged elections, we have open borders," he said.

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Biden, meanwhile, was hoping to inject star power and hard cash into his battle. Hollywood stars joined Obama for the gala evening that Biden's campaign says has already raised $28 million.

"It's the largest Democratic fundraiser ever," Clooney said.

Outside the fundraiser, groups of pro-Palestinian activists protesting Biden's support for Israel's assault on Gaza were kept away by a phalanx of police in riot gear.

Obama meanwhile took the stage with Biden, his former vice president, for a conversation moderated by late-night comic Jimmy Kimmel.

"We have someone to worry about," Obama told donors, referring to Trump. "And there's a whole agenda that we should be concerned about. But we can take pride in affirming the extraordinary work that Joe has done."

Biden's turn with the stars means he skipped a huge international peace conference for Ukraine Saturday in Switzerland, with Vice President Kamala Harris attending instead.