Something the nation has never seen before began playing out in a Denver courtroom this morning: a trial to determine whether a major party’s likely presidential nominee is eligible to be president at all.

The lawsuit, filed in September by six Colorado voters with the help of a watchdog group, Citizens for Responsibility and Ethics in Washington, argues that former President Donald Trump is ineligible to hold office again under Section 3 of the 14th Amendment. That section disqualifies anyone who “engaged in insurrection or rebellion” against the Constitution after having taken an oath to support it.

The plaintiffs say Trump’s efforts to overturn the 2020 election — namely his actions before and while his supporters stormed the Capitol on Jan. 6, 2021, to try to stop the certification of Joe Biden’s victory — meet the disqualification criteria.

Sarah B. Wallace, the state district court judge presiding over the case, rejected multiple requests from Trump and from the Colorado Republican State Central Committee in recent weeks to dismiss the case without a trial.

Wallace also on today rejected a call for her to recuse herself because she had donated to an organization that opposes Republican candidates in Colorado — a donation that Trump’s presidential campaign was fervently highlighting. She said she had not formed any opinion on the legal issues in the case.

In an opening statement today, Eric Olson, a lawyer for the plaintiffs, said his team would argue that Jan. 6 was “an insurrection against the Constitution” and that Trump engaged in that insurrection.

Olson ran through the familiar account of Trump’s speech to his supporters on the morning of Jan. 6 and the storming of the Capitol that followed. He reminded the court that Trump did not call off the mob for hours.

“We are here because Trump claims, after all that, he has the right to be president again,” he said. “But our Constitution, our shared charter of our nation, says he cannot do so.”

A lawyer for Trump, Scott Gessler, said his team would argue that “engaging” in an insurrection required more than “mere incitement through words.” He said Trump had never called for violence and had urged his supporters to act “peacefully and patriotically” — words that had been noted preemptively by Olson.

Olson said that using the phrase once did not outweigh the balance of a speech that repeatedly called on people to “fight.”

On Trump’s side, Gessler argued that it would be “anti-democratic” to deny him the ability to run and unprecedented for a court to deem a presidential candidate ineligible under the 14th Amendment.

If the legal analysis is at all close, Gessler said, “the rule of democracy says we err on the side of letting people vote.”

Wallace has laid out nine topics to be addressed at the trial, which is scheduled to last all week. They include whether Section 3 of the 14th Amendment applies to presidents; what “engaged” and “insurrection” mean under that section; whether Trump’s actions fit those definitions; and whether the amendment is “self-executing” — in other words, whether it can be applied without specific action by Congress identifying whom to apply it to.

These questions have been debated since the Jan. 6 attack, especially since Trump announced that he was running for president again, but there is little precedent to help answer them. The 14th Amendment was ratified shortly after the Civil War, and the disqualification clause was originally applied to people who had fought for the Confederacy. The courts have rarely had occasion to assess its modern application, and never in a case of this magnitude.

Some prominent constitutional experts — including conservative law professors William Baude and Michael Stokes Paulsen in an academic article, and conservative former Judge J. Michael Luttig and liberal law professor Laurence H. Tribe in The Atlantic — have argued that the clause applies to Trump.

But that view is far from universal among legal scholars, and several have told The New York Times over the past few months that the questions are complicated.

The court’s list of topics also calls for discussion of Section 3 of the 20th Amendment, which governs what happens if a new president and vice president have not “qualified” by the time they are supposed to take office.

The section says, in part, that “Congress may by law provide for the case wherein neither a president-elect nor a vice president-elect shall have qualified.”

Trump’s lawyers say this means that only Congress can enforce constitutional qualifications for the presidency. Lawyers for the plaintiffs rejected that argument in a brief last week, saying the “plain language” of the amendment — which refers to the “president-elect” — applies only to a person whom has already been elected and has nothing to do with states’ ability to adjudicate candidates’ qualifications.

The Colorado lawsuit is one of several efforts around the country to remove Trump from ballots under the 14th Amendment. Oral arguments in a case in Minnesota are scheduled to begin Thursday, and lawsuits have also been filed in New Hampshire and Michigan. Separately, Democratic legislators in California asked their state’s attorney general last month to seek a court opinion on Trump’s eligibility.

Whatever verdicts come in these cases will not be final. They will almost certainly be appealed by the losing side, and the Supreme Court — which has a 6-3 conservative majority, including three justices appointed by Trump — is likely to have the final say.

This article originally appeared in The New York Times.