Former US president Donald Trump cannot appear on the ballot in Colorado in next year's presidential election due to his role in the 6 January 2021 attack on the US Capitol by his supporters, the state's top court ruled.
The court ruled that Mr Trump cannot appear on the state's ballot papers in the next presidential election - or the primary election, where candidates must be certified by 5 January.
Mr Trump said he will appeal the ruling.
The justices of the Colorado Supreme Court - all of them Democrat appointees - ruled four to three in favour of banning Mr Trump from standing for election in the state, in an action brought by activists seeking Mr Trump's exclusion.
The majority accepted an interpretation of the 14th amendment to the US constitution - brought in after the American Civil War to ban officials who had supported the confederacy from holding federal office - that Mr Trump's actions leading up to and including the attack on the Capitol constituted an insurrection against the US, and thus debars him from office.
The ruling applies only to Colorado's 5 March Republican primary but it could affect Mr Trump's status in the state for the 5 November general election.
Nonpartisan US election forecasters view Colorado as safely Democratic, meaning that President Joe Biden will likely carry the state regardless of Mr Trump's fate there.
Mr Trump vowed to appeal the ruling to the US Supreme Court, and the Colorado court said it would delay the effect of its decision until at least 4 January to allow for an appeal.
The ruling sets the stage for the Supreme Court, whose 6-3 conservative majority includes three Trump appointees, to consider whether Mr Trump is eligible to serve another term as president.
The Colorado court concluded that the US constitution bars Mr Trump, the frontrunner for the Republican nomination in 2024, from appearing on the ballot because of his role instigating violence at the Capitol as politicians met to certify the results of the 2020 election.
The court's majority acknowledged the decision was "uncharted territory".
"We do not reach these conclusions lightly," the majority justices 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 favour, and without being swayed by public reaction to the decisions that the law mandates we reach."
Mr Trump's campaign called the court decision "undemocratic".
"The Colorado Supreme Court issued a completely flawed decision tonight and we will swiftly file an appeal to the United States Supreme Court," a spokesperson from the Trump campaign said.
The decision reverses a ruling by a lower court judge who found Mr Trump engaged in insurrection by inciting his supporters to violence, but concluded that, as president, Mr Trump was not an "officer of the United States" who could be disqualified under the amendment.
The Biden campaign declined to comment.
Read more: What does Trump's disqualification ruling in Colorado mean?
The case was brought by a group of Colorado voters, aided by the advocacy group Citizens for Responsibility and Ethics in Washington (CREW), who argued that Mr Trump should be disqualified for inciting his supporters to attack the Capitol in a failed attempt to obstruct the transfer of presidential power to Mr Biden after the 2020 election.
CREW President Noah Bookbinder said in a statement that the court's decision is "not only historic and justified, but is necessary to protect the future of democracy in our country."
Courts have rejected several lawsuits seeking to keep Mr Trump off the primary ballot in other states.
Minnesota's top court rebuffed an effort to disqualify Mr Trump from the Republican primary in that state, but did not rule on his overall eligibility to serve as president.
Some advocates had hoped the Colorado case would boost the overall disqualification effort and potentially put the issue before the US Supreme Court.
Mr Trump's campaign has condemned 14th Amendment challenges as an attempt to deny millions of voters their preferred choice for president.
His lawyers argued that his speech to supporters on the day of the riot was protected by his right to free speech, adding that the constitutional amendment does not apply to US presidents and that Congress would need to vote to disqualify a candidate.
Three Colorado Supreme Court justices dissented from yesterday's ruling.
One of the dissenting justices, Carlos Samour, said in a lengthy opinion that a lawsuit is not a fair mechanism for determining Mr Trump's eligibility for the ballot because it deprives him of his right to due process, noting that a jury has not convicted him of insurrection.
"Even if we are convinced that a candidate committed horrible acts in the past - dare I say, engaged in insurrection - there must be procedural due process before we can declare that individual disqualified from holding public office," Judge Samour said.
Additional reporting Reuters
