The Colorado Supreme Court on Tuesday declared former President Donald Trump ineligible for the White House under the U.S. Constitution’s insurrection clause and removed him from the state’s presidential primary ballot, setting up a likely showdown in the nation’s highest court to decide whether the front-runner for the GOP nomination can remain in the race.

The decision from a court whose justices were all appointed by Democratic governors marks the first time in history that Section 3 of the 14th Amendment has been used to disqualify a presidential candidate.

“A majority of the court holds that Trump is disqualified from holding the office of president under Section 3 of the 14th Amendment,” the court wrote in its 4-3 decision.

Colorado’s highest court overturned a ruling from a district court judge who found that Trump incited an insurrection for his role in the Jan. 6, 2021, attack on the Capitol, but said he could not be barred from the ballot because it was unclear that the provision was intended to cover the presidency.

The court stayed its decision until Jan. 4, or until the U.S. Supreme Court rules on the case.

“We do not reach these conclusions lightly,” wrote the court’s majority. “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.”

Trump’s attorneys had promised to appeal any disqualification immediately to the nation’s highest court, which has the final say about constitutional matters. His campaign said it was working on a response to the ruling.

Trump lost Colorado by 13 percentage points in 2020 and doesn’t need the state to win next year’s presidential election. But the danger for the former president is that more courts and election officials will follow Colorado’s lead and exclude Trump from must-win states.

Colorado officials say the issue must be settled by Jan. 5, the deadline for the state to print its presidential primary ballots.

Dozens of lawsuits have been filed nationally to disqualify Trump under Section 3, which was designed to keep former Confederates from returning to government after the Civil War. It bars from office anyone who swore an oath to “support” the Constitution and then “engaged in insurrection or rebellion” against it, and has been used only a handful of times since the decade after the Civil War.

The Colorado case is the first where the plaintiffs succeeded. After a weeklong hearing in November, District Judge Sarah B. Wallace found that Trump indeed had “engaged in insurrection” by inciting the Jan. 6 attack on the Capitol, and her ruling that kept him on the ballot was a fairly technical one.

Trump’s attorneys convinced Wallace that, because the language in Section 3 refers to “officers of the United States” who take an oath to “support” the Constitution, it must not apply to the president, who is not included as an “officer of the United States” elsewhere in the document and whose oath is to “preserve, protect and defend” the Constitution.

The provision also says offices covered include senator, representative, electors of the president and vice president, and all others “under the United States,” but doesn’t name the presidency.

The state’s highest court didn’t agree, siding with attorneys for six Colorado Republican and unaffiliated voters who argued that it was nonsensical to imagine the framers of the amendment, fearful of former Confederates returning to power, would bar them from low-level offices but not the highest one in the land.

“You’d be saying a rebel who took up arms against the government couldn’t be a county sheriff, but could be the president,” attorney Jason Murray said in arguments before the court in early December.

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Original: Fortune | FORTUNE: Colorado’s Supreme Court takes extraordinary step of banning Donald Trump from state’s primary ballot after ruling he ‘engaged in insurrection’