Donald Trump speaks during an event.

Donald Trump is ineligible for the US presidency because of his involvement in the January 2021 assault on the Capitol, Colorado's supreme court ruled Tuesday, setting off a political earthquake that could upend next year's election.

The stunning legal decision — which Trump's campaign said it would appeal — drew immediate condemnation from Republicans across the spectrum, and looked set to light a fire under the former reality TV star's claim to political persecution.

The ruling, which only applies to the Colorado primary ballot, is the first of a number of legal actions across the country to successfully invoke the US Constitution's 14th Amendment, which bars from office anyone formerly sworn to protect the country who later engages in insurrection.

"A majority of the court holds that President Trump is disqualified from holding the office of President under Section Three of the Fourteenth Amendment to the United States Constitution," the Colorado high court wrote.

"Because he is disqualified, it would be a wrongful act under the Election Code for the Colorado Secretary of State to list him as a candidate on the presidential primary ballot.

"We do not reach these conclusions lightly," the 4-3 majority wrote.

"We are mindful of the magnitude and weight of the questions now before us. We are likewise mindful of our solemn duty to apply the law, without fear or favor, and without being swayed by public reaction to the decisions that the law mandates we reach."

An earlier ruling by a lower court found that while Trump had clearly given succor to the January 6 riot, the office of president was not included in the list of federal elected positions affected by the 14th Amendment.

Noah Bookbinder of campaign group Citizens for Responsibility and Ethics in Washington, which brought the original case, took to social media to hail Tuesday's ruling, calling it "a huge moment for democracy."

"It is not only historic and justified, but is necessary to protect the future of democracy in our country.

"Our Constitution clearly states that those who violate their oath by attacking our democracy are barred from serving in government."

Agence France-Presse