The US Supreme Court appeared sceptical of Colorado's move to bar Donald Trump from the state's presidential primary, during tough questioning on Thursday.
The ex-president was removed from the ballot by Colorado's top court in December under the 14th Amendment.
At the hearing, justices grilled the attorney defending the move about its constitutionality, its real-world consequences and defining "insurrection".
It is unclear when the court will rule.
Mr Trump, who did not attend the hearing, remains the overwhelming favourite to clinch the Republican nomination for president and set-up a rematch with President Joe Biden in November.
The legal challenge hinges on a Civil War-era constitutional amendment that bans anyone who has "engaged in insurrection or rebellion" from holding federal office.
In its judgement in December, the Colorado Supreme Court said Mr Trump's actions during the 2021 Capitol riot amounted to insurrection, and the state's attorney repeated that claim on Thursday.
Most of the tough questioning went to lawyer Jonathan Murray, representing the five Coloradoans who originally sued to kick Mr Trump from the ballot.
One of the three court members nominated by Mr Trump, conservative Justice Brett Kavanaugh, challenged him about the potential "disenfranchising effect" of kicking Mr Trump from the ballot, by not allowing citizens to cast their votes as they wanted.
Mr Murray said "the reason we're here is that President Trump tried to disenfranchise 80 million Americans who voted against him" during the violence on 6 January 2021, where rioters tried to stop congress from certifying that Mr Biden won the 2020 election.
Mr Trump's team fought back against that allegation, telling justices that the event was "a riot, not an insurrection".
"The events were shameful, criminal, violent, all of those things but did not qualify as an insurrection as that term is used in Section 3," Mr Trump's attorney Jason Mitchell told the court.
Justices on both wings of the court appeared reluctant to uphold the Colorado ban, subjecting Mr Murray to a barrage of complex legal queries as the morning went on.
Chief Justice John Roberts observed that if the court upheld the Colorado ruling it could unleash chaos on the US political system by granting states the unilateral power to strike candidates from the ballot.
"It will come down to just a handful of states that are going to decide the presidential election. That's a pretty daunting consequence," he said.
And Justice Elena Kagan appeared to agree, noting: "I think that the question that you have to confront is why a single state should decide who gets to be president of the United States."
Enforcing the 14th amendment would have "to be federal, national means," she suggested.
But Trump's team were also subjected to a grilling by the justices, who challenged attorney Jonathan Mitchell on his claims that the 14th Amendment didn't apply to the presidency.
Mr Trump's team have long insisted that the term "officer of the United States" outlined in the provision can only apply to an appointed official, not the elected president.
Justice Amy Comey Barrett, a conservative also appointed to the court by Mr Trump, expressed a heavy scepticism when examining the Trump team's claims that the 14th Amendment did not apply to the presidency.
Nonetheless, it appears unlikely that the court will uphold the initial Colorado ruling.
Robert Tsai, a Boston University constitutional law professor, told the OceanNewsUK that the state's ruling "is toast".
The argument that Prof Tsai said most justices seemed to be attracted to was the idea that the power to remove presidential candidate from a ballot under Section 3 belonged to congress, not the states.
The court has not said when it will issue its decision, but it is expected soon. The court expedited the case and is under pressure to rule before 5 March, when Colorado holds its primary.
