- The strategy involves trying to block Mr Trump from the primary ballot by invoking a rarely used provision of the US Constitution - Section 3 of the 14th Amendment - that bars those who have "engaged in insurrection or rebellion" against the country from holding federal office.
- Initially backed by liberal activists, the theory gained more prominence in recent months as some conservatives also embraced it.
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A longshot legal bid in multiple US states to disqualify Donald Trump from the 2024 US presidential ballot has pulled off a shock victory in Colorado.
The strategy involves trying to block Mr Trump from the primary ballot by invoking a rarely used provision of the US Constitution - Section 3 of the 14th Amendment - that bars those who have "engaged in insurrection or rebellion" against the country from holding federal office.
Initially backed by liberal activists, the theory gained more prominence in recent months as some conservatives also embraced it.
The Colorado Supreme Court was the first to side with the theory, removing Mr Trump from the state's 2024 presidential ballot in a ruling published the week before Christmas.
It is the first time that Section 3 of the 14th Amendment has been used to disqualify a presidential candidate.
The decision is on hold until January pending appeal and does not apply to states outside of Colorado. Several attempts in other states to kick Mr Trump off the ballot have also failed until now.
Critics have warned that, if such cases move forward, it risks robbing voters of the right to deliver their own verdict on whether the former president should return to the White House.
Similar challenges in three other states have already stalled. A lawsuit in New Hampshire was dismissed; a judge in Michigan ruled it was "a nonjusticiable, political" issue that should be decided by Congress, not the state; and a court in Minnesota rebuffed the attempt ahead of the primary election but left the door open for petitioners to file another challenge on the general election ballot.
The untested legal gambit is a last-ditch effort to bar the candidacy of an ex-president who remains popular with his base.
Its ultimate arbiter will likely be the conservative Supreme Court that Mr Trump helped shape. In the Colorado case, it appears his campaign aims to bring their arguments to the nation's ultimate legal arbiter.
What is the theory?
The 14th Amendment was ratified after the American Civil War, and Section 3 was deployed to bar secessionists from returning to previous government posts once southern states re-joined the Union.
It was used against the likes of Confederate president Jefferson Davis and his vice-president Alexander Stephens, both of whom had served in Congress, but has seldom been invoked since.
It re-emerged as a political flashpoint in the wake of Mr Trump's effort to overturn his 2020 election defeat, which culminated in the riot at the US Capitol in January 2021.
In the riot's aftermath, the US House of Representatives impeached the then-president on a charge of "incitement of insurrection".
Had the US Senate voted to convict him, it would have had the option to take a second, simple-majority vote to bar him from ever serving in office again.
But that never happened: the Senate failed to reach the two-thirds majority required to convict Mr Trump, so there was no second vote.
Does Section 3 apply to Trump?
Free Speech For People, an advocacy group, has been arguing that it does.
Last year, the group filed challenges against five Trump-backing lawmakers whom it labelled "insurrectionists".
One - against Georgia congresswoman Marjorie Taylor Greene - was heard in court but ultimately defeated.
The 14th Amendment was not written solely to apply to the post-Civil War era, but also to future insurrections, argues Ron Fein, the organisation's legal director.
He told the BBC the US Capitol riot succeeded "in delaying the peaceful transfer of power for the first time in our nation's history, which is further than the Confederates ever got".
"The particular candidates we challenged in 2022 had participated or assisted in the efforts that led up to the insurrection," Mr Fein said.
All these cases, he argued, established important legal precedents that can be applied to show "Trump is the chief insurrectionist".
In New Mexico, a challenge brought by the Citizens for Responsibility and Ethics in Washington (Crew) watchdog group saw Couy Griffin, a local county commissioner who participated in the Capitol riot, removed from office under Section 3 - the first such ruling since 1869.
How will it move forward?
The case in Colorado challenging Mr Trump's eligibility was filed by Crew on behalf of six state residents.
Despite its failed attempts in Michigan and Minnesota, Free Speech For People has indicated it will bring future challenges. Its success in Colorado will probably fuel that effort further.
The group is also separately petitioning the top election officials in at least 18 states to remove Mr Trump from the primary ballot.
Each action already has drawn, or will inevitably draw, an objection from the candidate himself - triggering a process that could ultimately place his fate in the hands of the US Supreme Court.
The legal strategy has picked up steam since August, when Mr Trump was accused of election subversion in two separate criminal cases.
That same month, conservative legal scholars William Baude and Michael Stokes Paulsen wrote in a law review paper that Section 3 is "self-executing, operating as an immediate disqualification from office, without the need for additional action by Congress".
Mr Trump could therefore be rendered ineligible for the ballot "by every official, state or federal, who judges qualifications", the pair concluded.
Mr Baude and Mr Paulsen are members of the Federalist Society, a highly influential conservative advocacy group.
They believe the Constitution must be interpreted as its authors intended at the time, and their stance has since been backed by other legal experts with conservative credentials.
Even the Supreme Court, with its conservative majority and trio of Trump-appointed judges, may be receptive to their argument, said Jeffrey Sonnenfeld, a dean at the Yale School of Management who supports the Baude-Paulsen perspective.
With Republican primary voters heading to the polls in less than two months, any case must be decided quickly.
What's the argument against it?
Detractors have questioned both the theory's viability, and whether it should even be implemented in a highly partisan America.
In an opinion piece for Bloomberg, liberal professor Noah Feldman wrote: "Donald Trump is manifestly unfit to be president. But it's up to voters to block him. Magic words from the past won't save us."
"To make a tortured legalistic logic to try to stop people from voting for who they want to vote for is a Soviet-style, banana republic argument," said New Hampshire Republican Party chairman Chris Ager.
"I'm not a Trump supporter. I'm neutral," he added. "But this whole attempt is bad for the country."
Even Brad Raffensperger, a Republican and the top election official in Georgia and a previous target of Mr Trump's ire, rejected the move as "merely the newest way of attempting to short-circuit the ballot box".
But in New Hampshire, the first state in the nation to cast ballots in the Republican primaries, a top Republican attorney who ran for the US Senate in 2020 with Mr Trump's endorsement takes a different view.
"To me, it's purely about the Constitution," said Bryant "Corky" Messner. "The US Constitution is more important than any one individual, be it Donald Trump or anyone else."
Mr Messner intends to finance any 14th Amendment challenges in his state and wants the courts to deliver their verdict before he can decide on whether to support Mr Trump.
"If he ends up being the nominee of the Republican Party and he's not disqualified, I'll vote for him," he said.
What does Trump say?
Despite his mounting legal troubles, Mr Trump remains the dominant frontrunner for the Republican nomination and is polling neck-and-neck with President Joe Biden ahead of their expected rematch.
The Trump campaign has said that the legal challenge is "stretching the law beyond recognition" and has no basis "except in the minds of those who are pushing it".
"Democrat Party leaders are in a state of paranoia over the growing, dominant lead President Trump has amassed in the polls," spokesman Steven Cheung said.
"They have lost faith in the failed Biden presidency and are now doing everything they can to stop the American voters from throwing them out of office next November."
Mr Trump's attorney in the Colorado case argued that the twin dismissals in Michigan and Minnesota were evidence of "an emerging consensus here across the judiciary".
"The petitioners are asking this court to do something that's never been done in the history of the United States," Scott Gessler said. "The evidence doesn't come close to allowing the court to do it."
The ruling by Colorado's Supreme Court that removed Mr Trump from the 2024 state ballot could reverse that narrative. It may have also left the former president regretting he did not say more in court, where his team challenged the case.
The state's top elections official, Jena Griswold, a Democrat, criticised Mr Trump in an interview for not wanting "to come and give their side of the story" at the hearing.
"He's not planning to testify, he's not giving deposition, and for someone who just loves to grandstand about the cases against him, his silence, compared to what his testimony would be under oath, is deafening," she told MSNBC.