Colorado's Supreme Court has ruled that Donald Trump cannot run for president next year in the state, citing a constitutional insurrection clause.
The court ruled 4-3 that Mr Trump was not an eligible candidate because he had engaged in an insurrection over the US Capitol riot nearly three years ago.
It does not stop Mr Trump running in the other states and his campaign says it will appeal to the US Supreme Court.
The decision, they said, was "completely flawed".
The ruling only mentions the state's primary election, external on 5 March, when Republican voters will choose their preferred candidate for president. But it could affect the general election in Colorado next November.
It is the first-ever use of Section 3 of the US Constitution's 14th Amendment to disqualify a presidential candidate. The measure was ratified after the American Civil War to block secessionists from returning to previous government roles once southern states re-joined the Union.
Tuesday's decision - which has been placed on hold pending appeal until next month - only applies in Colorado. Similar attempts to kick Mr Trump off the ballot in New Hampshire, Minnesota and Michigan have failed.
Voters react to Trump being kicked off Colorado's ballot
The justices wrote in their ruling: "We do not reach these conclusions lightly. 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."
One of the three justices who dissented, Carlos Samour, though, said the government could not "deprive someone of the right to hold public office without due process of law".
The decision reverses an earlier one from a Colorado judge, who ruled the ban did not apply to presidents but found Mr Trump had participated in an insurrection. The former president's supporters stormed Congress on 6 January 2021 while lawmakers were certifying President Joe Biden's election victory.
The decision does not go into effect until 4 January 2024, coming right up to the deadline for printing the state's presidential primary ballots.
That delay could be extended if the Supreme Court takes up the case, which legal experts speaking to the BBC expect it to do.
They also expect the Colorado ruling to have a tough time in the highest court in the land, where conservatives hold a 6 to 3 majority.
The Supreme Court will likely not uphold the disqualification of Mr Trump out of "institutional caution", said Professor Samuel Issacharoff, a constitutional law professor at New York University.
"It cannot be that the national candidacy for presidency is determined on a state by state basis," he said. "That would be a breakdown of democratic order."
The Supreme Court is also wary of state courts when it comes to the election, said Robert Tsai, a constitutional law professor from Boston University.
The last time it heard a case involving a presidential election was Bush v Gore in 2000, when it stopped the state of Florida from carrying out a vote recount.
"Since then, the court has become even more hostile or sceptical of state court judges doing things to interfere with the election of a president," he said.
The Trump campaign and fellow Republicans, including other candidates in the primary, accused the Colorado court of playing politics.
Calling the ruling "completely flawed", campaign spokesman Steven Cheung lambasted the justices, who were all appointed by Democratic governors.
"Democrat Party leaders are in a state of paranoia over the growing, dominant lead President Trump has amassed in the polls," Mr Cheung said in a statement.
The Colorado Republican party said it would withdraw from the state's primary process if the ruling was allowed to stand and one of Mr Trump's rivals, Vivek Ramaswamy, pledged to withdraw his name from the ballot it Mr Trump was blocked.
Speaking to reporters on Wednesday, President Biden said it is up to the courts to decide if Mr Trump qualifies, but added there's "no question" his predecessor supported an insurrection.
Mr Trump, the current frontrunner for the Republican nomination, lost Colorado by a wide margin in the last presidential election. But if courts in more competitive states followed suit on Tuesday's ruling, his White House bid could face serious problems.
During a one-week trial in the state last month, his lawyers argued he should not be disqualified because he did not bear responsibility for the US Capitol riot.
But in its ruling, the court majority said Mr Trump's messages before the riot were a "call to his supporters to fight and… his supporters responded to that call".
Mr Trump is facing four criminal cases, including one federal and one state case in Georgia related to the his attempt to overturn the 2020 election.
