By Andrew Chung and John Kruzel

WASHINGTON (Reuters) – U.S. Supreme Court justices on Thursday questioned a lawyer for Donald Trump as the former president fights to avoid being kicked off state presidential ballots for his actions involving the 2021 Capitol attack in a case with major implications for the November election.

The nine justices were hearing his appeal of a Dec. 19 ruling by Colorado’s top court to disqualify Trump from the state’s Republican primary ballot under the U.S. Constitution’s 14th Amendment after finding that he participated in an insurrection.

Section 3 of the 14th Amendment bars from holding public office any “officer of the United States” who took an oath “to support the Constitution of the United States” and then “engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.”

The justices grilled Trump’s lawyer Jonathan Mitchell at the outset of the arguments.

Trump was not present. Instead, he planned to start his day at his Florida home and travel to Nevada, according to a source familiar with his plans. Nevada on Thursday night holds a nominating caucus that Trump is expected to win handily as he cruises towards his party’s nomination to challenge Democratic President Joe Biden on Nov. 5.

Dozens of demonstrators gathered outside the white marble courthouse on a sunny and chilly winter morning. Some held signs reading, “Failed Coup,” “Remove Trump” and “Trump is a Traitor.” Police set up barricades around the courthouse for security.

In an interview on Thursday morning with a conservative radio show host, Trump said allies of President Joe Biden were trying to use the Colorado case to take him out of the presidential contest.

“They want to have the Supreme Court rule or vote to take me out of the race. That would be a very terrible thing to do,” Trump said. “It’s about the vote. It’s about our Constitution.”

The case calls on the Supreme Court to play a central role in a presidential contest unlike any since its landmark Bush v. Gore decision that handed Republican George W. Bush the presidency over Democrat Al Gore in 2000.

Anti-Trump forces have sought to disqualify him in more than two dozen other states – a mostly unsuccessful effort – over his actions relating to the Jan. 6, 2021, Capitol attack. Maine also has barred him from its ballot, a decision put on hold pending the Supreme Court’s ruling in the Colorado case.

Colorado’s Republican primary is scheduled for March 5. The plaintiffs who sued seeking to disqualify Trump are four Republican voters and two unaffiliated voters.

The Supreme Court’s 6-3 conservative majority includes three justices appointed by Trump.

Trump’s lawyers have argued he is not subject to the disqualification language because a president is not an “officer of the United States,” that the provision cannot be enforced by courts absent congressional legislation, and that he did not engage in an insurrection.

The 14th Amendment was ratified following the American Civil War of 1861-1865 in which seceding Southern states that allowed the practice of slavery rebelled against the U.S. government.

Trump supporters attacked police and swarmed the Capitol in a bid to prevent Congress from certifying Biden’s victory. Trump gave an incendiary speech to supporters beforehand, telling them to go to the Capitol and “fight like hell.” He then for hours rebuffed requests that he urge the mob to stop.

The plaintiffs have said a president clearly is an “officer of the United States” because “it would make no sense to read Section 3 as disqualifying all oath-breaking insurrectionists except the one holding the highest office in the land.”

The justices also may soon confront another Trump-related case. Trump faces a Monday deadline to ask the Supreme Court to intervene after a U.S. appeals court rejected his claim for immunity in one of two cases in which he faces criminal charges related to his efforts to overturn his 2020 election loss to Biden.

The justices could resolve the case without explicitly deciding whether Trump engaged in an insurrection. The case also differs sharply from the criminal cases against him. The eventual ruling in the Colorado case, even if favorable to Trump, may not indicate how the justices would rule on his bid for immunity from prosecution as a former president.

The plaintiffs in the Colorado case are backed by Citizens for Responsibility and Ethics in Washington, a liberal watchdog group.