Milwaukee, Wisconsin - President Joe Biden on Wednesday branded Donald Trump an insurrection supporter after Colorado's top court blocked the Republican from seeking a second term over the 2021 Capitol assault.

"I'll let the court make that decision. But he certainly supported an insurrection. No question about it, none, zero," Biden told reporters after arriving in Milwaukee, Wisconsin aboard Air Force One.

"It's self-evident. You saw it all," added the 81-year-old Democrat, who is likely to face Trump in a close-fought 2024 presidential election in a rematch of their 2020 battle.

While Biden took pains to stress that the matter was up to the courts, it marked a rare occasion that he has directly commented on the multiple legal cases piling up against Trump.

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The ex-president's campaign has said it will appeal to the Supreme Court against the Colorado court's ruling, which only applies to the primary ballot in the state.

It is the first of multiple legal actions across the country to successfully invoke the Constitution's 14th Amendment, which bars from office anyone sworn to protect the country who later engages in insurrection.

Neck-and-neck with Trump in the polls and suffering from low approval ratings, Biden has taken off the gloves to launch direct rhetorical attacks on Trump.

Biden told a campaign reception on Tuesday near Washington that "the greatest threat Trump poses is to our democracy. Because if we lose, we lose everything."

He described Trump as "sitting there, watching it unfold on TV as a mob attacked the Capitol" in the assault by the Republican's supporters on January 6, 2021, aimed at overturning Trump's loss to Biden.

"This is a former president who actually condones and encourages violence against his fellow Americans," he said. "It's despicable."