As President Trump dominates the Republican presidential primary, some liberal groups and legal experts contend that a rarely used clause of the Constitution prevents him from being president after the January 6, 2021, attack on the Capitol.

The 14th Amendment bars from office anyone who once took an oath to uphold the Constitution but then “engaged” in “insurrection or rebellion” against it. 

A growing number of legal scholars say the post-Civil War clause applies to Mr. Trump after his role in trying to overturn the 2020 presidential election and encouraging his backers to storm the U.S. Capitol.

Two liberal nonprofits pledge court challenges should states’ election officers place Mr. Trump on the ballot despite those objections.

The effort is likely to trigger a chain of lawsuits and appeals across several states that ultimately would lead to the U.S. Supreme Court, possibly in the midst of the 2024 primary season. The matter adds even more potential legal chaos to a nomination process already roiled by the front-runner facing four criminal trials.

Now Mr. Trump’s very ability to run could be litigated as Republicans are scheduled to start choosing their nominee, starting with the Iowa caucuses on January 15.

“There’s a very real prospect these cases will be active during the primaries,” said a law professor at Indiana University, Gerard Magliocca, warning that there could be different outcomes in different states before the Supreme Court makes a final decision. “Imagine you have an opinion that says he’s not eligible and then there’s another primary where he’s on the ballot.”

Though most litigation is unlikely to begin until October, when states begin to set their ballots for the upcoming primary, the issue has gotten a boost from a recently released law review article written by two prominent conservative law professors, William Baude and Michael Paulsen. They concluded that Mr. Trump must be barred from the ballot due to the clause in the third section of the 14th Amendment.

That section bars anyone from Congress, the military, and federal and state offices if they previously took an oath to support the Constitution and “have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.”

In their article, scheduled to be published in the University of Pennsylvania Law Review, Messrs. Baude and Paulsen said they believe the meaning is clear.

“Taking Section Three seriously means excluding from present or future office those who sought to subvert lawful government authority under the Constitution in the aftermath of the 2020 election,” they write.

The issue came up during last week’s Republican presidential debate in Milwaukee, when Governor Hutchinson warned that “this is something that could disqualify him under our rules and under the Constitution.”

In 2021, the nonprofit Free Speech For People sent letters to the top election official in all 50 states requesting Mr. Trump’s removal if he were to run again for the presidency. The group’s legal director, Ron Fein, noted that after years of silence, officials are beginning to discuss the matter.

“The framers of the 14th Amendment learned the bloody lesson that, once an oath-breaking insurrectionist engages in insurrection, they can’t be trusted to return to power,” Mr. Fein said.

Ahead of the 2022 midterms, the group sued to remove Representatives Marjorie Taylor-Greene and Madison Cawthorn, both Republicans, from the ballot over their support for the January 6 protest. The judge overseeing Ms. Greene’s case ruled in her favor, while Mr. Cawthorn’s case became moot after he was defeated in his primary.

The complex legal issues were highlighted on Wednesday when the Arizona Republic reported that Secretary of State Adrian Fontes said his hands are tied because of a ruling by that state’s high court that only Congress can disqualify someone on Arizona’s presidential ballot. Mr. Fontes, a Democrat, called the ruling “dead, flat wrong” in an interview with the Republic but said he would abide by it.

If Mr. Trump appears on the Arizona ballot, those who believe he’s not qualified can still sue in federal court to remove him.

Other secretaries of state are warily navigating the legal minefield.

In a radio interview earlier this week, Michigan’s secretary of state, Jocelyn Benson, a Democrat, said “there are valid legal arguments being made” for keeping Mr. Trump off the ballot and that it’s something she is discussing with other secretaries of state, including those in presidential battlegrounds.

The Republican secretary of state in Georgia, Brad Raffensperger, who withstood pressure from Mr. Trump when he sought to overturn the 2020 results in the state, suggested the issue should be up to voters.

“As Georgia’s Secretary of State, I have been clear that voters are smart and deserve the right to decide elections,” he said in an emailed statement.

Mr. Trump argues that any effort to prevent him from appearing on a state’s ballot amounts to “election interference” — the same way he is characterizing the criminal charges filed against him at New York and Atlanta and by federal prosecutors at Washington, D.C., and Florida.

“And I think what’s happening is there’s really been a backlash against it,” Mr. Trump told the conservative channel Newsmax.

Indeed, the New Hampshire secretary of state’s office was flooded with messages about the issue on Monday, said spokeswoman Anna Sventek. Earlier in the day, a conservative personality had falsely claimed the state was about to strike Mr. Trump from the ballot.

On Wednesday, a long-shot Republican presidential candidate, John Anthony Castro, of Texas, filed a complaint in a New Hampshire court contending the 14th Amendment barred Mr. Trump from that state’s ballot.

The eventual, bigger court challenges are expected to draw greater legal fire power. But a conservative law professor at Stanford University, Michael McConnell, who is not a Trump supporter, said the case is no slam dunk.

Mr. McConnell questions whether the provision even applies to the presidency because it is not one of the offices specifically listed in the 14th Amendment — which instead refers to “elector of president and vice president.” 

He also said it’s unclear whether the January 6 attack constitutes an “insurrection” under the law or simply a less legally fraught incident such as a riot.

But Mr. McConnell also worries about the political precedent if Trump is ultimately removed from any state ballot.

“It’s not just about Trump. Every election where someone says something supportive of a riot that interferes with the enforcement of laws, their opponents are going to run in and try to get them disqualified,” he said.

Ratified in 1868, the 14th Amendment helped ensure civil rights for freed slaves — and eventually for all people in America — but also was used to prevent former Confederate officials from becoming members of Congress and taking over the government they had just rebelled against.

The clause allows Congress to lift the ban, which it did in 1872 as the political will to continue to bar former Confederates dwindled. The provision was almost never used after that. 

In 1919, Congress refused to seat a socialist in Congress, contending he gave aid and comfort to the country’s enemies during World War I. Last year, in the provision’s first use since then, a New Mexico judge barred a rural county commissioner who had entered the Capitol on January 6 from office under the clause.

If any state bars Mr. Trump from running, his reelection campaign is expected to sue, possibly taking the case directly to the Supreme Court. If no state bans him, Free Speech For People and another nonprofit, Citizens for Responsibility and Ethics in Washington, would likely challenge his presence on the ballot.

It’s critical that the high court settle the issue before the general election, said a law professor at Ohio State University, Edward Foley. His fear is that if Mr. Trump’s qualifications are not resolved and he wins, Democrats could try to block his ascension to the White House on January 6, 2025, triggering another democratic crisis.

Those pushing to invoke the amendment agree and say they think the case is clear.

“This isn’t a punishment. It’s like saying a president needs to be 35 years old and a natural born citizen,” the president of Citizens for Responsibility and Ethics in Washington, Noah Bookbinder, said. “You also need not to have helped organize an uprising against the government.”

Associated Press