Rulings in Colorado and Maine banning the ex-president from their primary nominating contests under the Constitution's "insurrection clause" set off a political earthquake that could upend the campaign.
But do they pose a genuine threat to Trump's presidential ambitions?
Why Trump might be ineligible
The US House of Representatives impeached the Republican tycoon for inciting insurrection after he called on his supporters to march on the US Capitol ahead of a deadly riot that delayed the certification of the 2020 election.
A large, bipartisan majority of the Senate agreed with the lower chamber's finding, but the tally fell short of the two-thirds of members required for a conviction, which would have barred Trump from seeking office again.
Federal prosecutors have since charged Trump with conspiracy over the violence, and he is scheduled for trial in March.
Meanwhile, activists have brought court challenges nationwide to stop his name appearing on ballots for state primaries under the 14th Amendment, which bars people from office if they have taken an oath to defend the Constitution but subsequently "engage in insurrection."
Where is Trump barred from running?
On December 19, the Colorado Supreme Court ruled that Trump was ineligible for the state's primary under the 14th Amendment's "insurrection clause."
Maine -- where the secretary of state, currently a Democrat, makes the initial eligibility determination -- followed suit on Thursday.
Related challenges have been filed nationwide, with decisions pending in 14 states, according to national security website Lawfare's online disqualification tracker.
What happens next?
Both states put their bans on hold while the legal process plays out -- meaning Trump will almost certainly appear on both ballots for their March 5 primaries.
Grassroots Republicans have challenged the ruling in Colorado and the Trump campaign has indicated that it intends to appeal in both cases.
The Maine decision would go to the state's Superior Court first, while Colorado's ruling has already been through the state system and would be elevated directly to the US Supreme Court.
What can the US Supreme Court do?
The strongly conservative-leaning high court -- which includes three Trump appointees -- can decline to review the Colorado case, meaning the Trump ban would stand.
In reality, most analysts believe that Maine joining with Colorado removes any doubt that the justices will act.
They would determine whether or not section three of the 14th Amendment -- the insurrection clause -- applies to a former president.
The court could also rule on whether the ban is automatic or would require an act of Congress, and whether the 2021 storming of the Capitol and Trump's role in it indeed qualify as insurrection.
The effect of the bans
Even if the Colorado and Maine bans stand, front-running Trump is expected to be able to secure the Republican nomination comfortably without those states, so his path to the White House would not necessarily be impeded.
But an adverse Supreme Court ruling that is binding on lower courts nationwide -- rather than a narrow procedural ruling affecting only Colorado -- could torpedo the ex-president's primary challenge.
He would then have to decide whether to run for president as a third-party candidate.
But even if the high court doesn't weigh in on the state level rulings and the damage is limited to Colorado and Maine, the justices could leave the door open to challenges to Trump's eligibility for the general election.
So he could well find himself barred from challenging President Joe Biden in November, even if he wins the Republican nomination.
Timetable
The gears of the US justice system grind notoriously slowly -- particularly at the top.
The Supreme Court has not indicated whether it intends to take up the Colorado appeal, nor how long it would take to rule on Trump's eligibility if it did. Trump's opponents have stressed the importance of an urgent decision.
The court can act quickly when required: Its ruling ending Florida's vote count in the 2000 election -- handing victory to George W. Bush -- took less than a month.
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