DENVER (AP) – Former President Donald Trump’s bid to win back the White House is now threatened by two sentences added to the U.S. Constitution 155 years ago.

The Colorado Supreme Court on Tuesday barred Mr. Trump from the state’s ballot under Section 3 of the 14th Amendment, which prohibits anyone who swore an oath to support the Constitution and then “engaged in insurrection” against it from holding office. It is the first time in history the provision has been used to prohibit someone from running for the presidency, and the U.S. Supreme Court is likely to have the final say over whether the ruling will stand.

If it does—which many legal experts say is a longshot—it is the end of Mr. Trump’s campaign because a Supreme Court decision would apply not just in Colorado, but to all states. It also could open a new world of political combat, as politicians in the future fish for judicial rulings to disqualify their rivals under the same provision.

Some conservatives have even considered using it against Vice President Kamala Harris, who raised bail money for those jailed during the violence following the murder of George Floyd in Minneapolis. They said that also should be considered an “insurrection” against the Constitution.

Here are some answers related to the 14th Amendment cases seeking to remove Mr. Trump from the ballot.

What Is the Impact of the Ruling?

So far, very little in the real world. Aware that the case was very likely going to the U.S. Supreme Court, the 4-3 Colorado Supreme Court majority stayed their own order until January 4—the day before the state’s primary ballots are due at the printer—or until the Supreme Court rules.

Technically, the ruling applies only to Colorado, and secretaries of state elsewhere are issuing statements saying Mr. Trump remains on the ballot in their state’s primary or caucus.

But it could embolden other states to knock Mr. Trump off the ballot. Activists have asked state election officials to do so unilaterally, but none have. Dozens of lawsuits have been filed, but all failed until Colorado.

The U.S. Supreme Court has never ruled on the meaning of Section 3. The justices can take the case as quickly as they like once Mr. Trump’s campaign files its appeal, which is not expected this week. The high court then could rule in a variety of ways—from upholding the ruling to striking it down to dodging the central questions on legal technicalities. But many experts warn that it would be risky to leave such a vital constitutional question unanswered.

“It is imperative for the political stability of the U.S. to get a definitive judicial resolution of these questions as soon as possible,” Rick Hasen, a law professor at the University of California, Los Angeles, wrote shortly after the ruling. “Voters need to know if the candidate they are supporting for president is eligible.”

What Will the U.S. Supreme Court Do?

It is always dangerous to try to predict a Supreme Court ruling. The high court is comprised of six justices appointed by Republicans, including three nominated by Mr. Trump himself. Partly because this is completely new legal ground, it is hard to predict how individual justices will rule based on their ideology.

Some of the strongest advocates of using Section 3 against Mr. Trump have been prominent conservative legal theorists and lawyers who argue that courts have to follow the actual words of the Constitution. Here, they argue, there’s no wiggle room—Mr. Trump is clearly disqualified.

The Colorado high court’s seven justices were all appointed by Democrats. But they split 4-3 on the ruling. The majority quoted a ruling from Neil Gorsuch, one of Mr. Trump’s conservative Supreme Court appointees, from when he was a federal judge in Colorado. He ruled then that the state properly kept a naturalized citizen born in Guyana off the presidential ballot because he didn’t meet the constitutional qualifications.

Courts are very hesitant to limit voters’ choices, however. There’s even a term for that—the “political question,” whether a legal dispute is better settled by the people the voters have selected to make the laws than by unelected judges. That is one reason all the other Section 3 lawsuits had failed so far.

Sometimes courts have dodged the essential question. That is what happened in Minnesota, where the state Supreme Court allowed Mr. Trump to stay on the ballot because, it found, the state party can place whomever it likes on its primary ballot. A Michigan appeals court came to the same conclusion. A New Hampshire judge dismissed a lawsuit by a little-known longshot Republican presidential candidate, saying the question of whether Mr. Trump belonged on the ballot was “non-justiciable.”

What Is Section 3 of the 14th Amendment?

Section 3 of the 14th Amendment was written to keep former confederates from returning to government office. It reads as follows.

“No person shall be a Senator or Representative in Congress, or elector of President and Vice-President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may by a vote of two-thirds of each House, remove such disability.”

The provision was used often in the years immediately after the Civil War, but fell into disuse after Congress granted an amnesty to many confederate veterans in 1872. The only record of it being used in the 20th century, according to legal scholars, was as justification in refusing to seat a socialist congressman in 1919 because he opposed U.S. involvement in World War I.

What Are Mr. Trump’s Legal Arguments?

The argument to disqualify Mr. Trump is that he clearly held an office under the United States, swore an oath and broke it in the January 6, 2021, attack on the U.S. Capitol. So he cannot return to office unless two-thirds of Congress lets him back in.

The arguments against disqualifying Mr. Trump are many. Mr. Trump’s lawyers have argued that, technically, the president isn’t an officer “under the United States”—that it is a legal term of art that refers to government appointees and therefore the provision doesn’t apply to him.

Even if it did, they have argued the January 6 attack was not an insurrection—it was more of a riot. And even if it was an insurrection, Mr. Trump did not “engage” in it—all he did was exercise his rights to free speech under the First Amendment. And state courts, the argument goes, are not in a position to determine whether January 6 was an insurrection—it would take months at least to hold a trial and get all the facts, and most witnesses are out of their jurisdiction.

Finally, they argue that even if the courts concluded January 6 was an insurrection and Mr. Trump was barred, that is not their decision to make—it is a political question for Congress.

What the Colorado Justices Said

The majority opinion said the Colorado Supreme Court did have jurisdiction to decide the matter, that the presidency was clearly an office in the United States and that Mr. Trump’s actions related to the Capitol attack fit the insurrection clause, in part because he urged his supporters during a rally beforehand to fight.

“President Trump asks us to hold that Section 3 disqualifies every oathbreaking insurrectionist except the most powerful one and that it bars oathbreakers from virtually every office, both state and federal, except the highest one in the land,” the court’s majority opinion said. “Both results are inconsistent with the plain language and history of Section 3.”

It is worth noting that three of the judges on the Colorado high court agreed with some of Mr. Trump’s arguments. They particularly chafed at the rushed and improvised nature of the groundbreaking case, which was heard by a district court judge in Denver judge in less than two months. That included a week of testimony from a handful of police and protesters who were at the January 6 attack, two constitutional law professors and experts on a president’s emergency powers and on right-wing political speech.

“I have been involved in the justice system for 33 years now, and what took place here doesn’t resemble anything I’ve seen in a courtroom,” Justice Carlos Samour wrote in a scathing dissent.

“If President Trump committed a heinous act worthy of disqualification, he should be disqualified for the sake of protecting our hallowed democratic system, regardless of whether citizens may wish to vote for him in Colorado,” Mr. Samour concluded. “But such a determination must follow the appropriate procedural avenues. Absent adequate due process, it is improper for our state to bar him from holding public office.”