The US Supreme Court will decide if Donald Trump can be kept off the presidential ballot because of his efforts to overturn his 2020 election loss.
A case will be heard in February on whether he can run nationwide after Colorado state moved to block him from appearing on the ballot in 2024.
The Supreme Court is hearing the case after Mr Trump sought to appeal the Colorado decision with other states also moving to disqualify him.
The former president remains a front runner for the Republican nomination despite facing legal challenges over allegedly inciting insurrection at the US capitol in 2021.
He is now stepping up his campaigning on what is the third anniversary of the January 6 riots, after which more than 1,230 were charged with federal crimes for their participation.
Mr Trump is accused of encouraging supporters to forcefully confront the state’s legislative command after refusing to accept defeat to Joe Biden in the 2020 election.
His legal challenge against the Colorado decision centres on whether a Civil War amendment will leave him ineligible to stand as it forbids anyone who has "engaged in insurrection or rebellion" from taking office.
The former president’s legal counsel said: "The Colorado Supreme Court decision would unconstitutionally disenfranchise millions of voters in Colorado and likely be used as a template to disenfranchise tens of millions of voters nationwide."
Mr Trump urged his supporters Friday evening not to be complacent in the face of a commanding polling lead as he kicked off the sprint to the Iowa caucuses with his first events of the election year.
"Ten days from now, the people of this state are going to cast the most important vote of your entire lives," Mr Trump told several hundred supporters gathered in Sioux Center. He implored them to turn out on caucus night, warning, "Bad things happen when you sit back."
Mr Trump held a pair of commit-to-caucus events, one in the far northwest corner of the state on the border with South Dakota and one in north-central Mason City. He will spend Saturday in Newton in central Iowa before heading to Clinton in the state's far east.
Mr Trump asked at one point in Sioux Center whether there was anyone in the friendly room who wasn't planning to vote for him, but then quickly warned them not to raise their hands.
"They're going to say he incited an insurrection," he said to laughs.
He repeated his claim that the 2020 election was stolen while also lashing out at President Biden who warned on Friday of the threat posed to democracy by Mr Trump.
The court's announcement came as President Joe Biden warned Mr Trump's efforts to retake theWhite House in 2024 pose a grave threat to the country.
Mr Biden said that January 6 2021 marked a moment where "we nearly lost America - lost it all".
He said this year's presidential race - a likely rematch with Mr Trump - is "all about" whether American democracy will survive.
