The Colorado ruling disqualifying Donald Trump from the ballot because he incited an insurrection on January 6 sets up another high-stakes, highly controversial political intervention by the US supreme court – a conservative-dominated panel to which Trump appointed three stringent rightwingers. Compromised in progressive eyes by those appointments and rulings including the removal of the federal right to abortion, the court was already due to decide whether Trump has immunity from prosecution regarding acts committed as president. Arising from one of four criminal indictments that have generated 91 charges, that case – concerning elected subversion if not incitement of insurrection – has produced intense scrutiny of Clarence Thomas, the longest-serving justice and a hardline conservative also at the centre of an ethics scandal. Thomas’s wife, Ginni Thomas, is a hard-right activist who was deeply involved in attempts to overturn Trump’s 2020 defeat by Joe Biden, a defeat which according to Trump’s lie was the result of electoral fraud. With the Colorado ruling, calls for Clarence Thomas to recuse from cases involving Trump will no doubt increase – and no doubt continue to be ignored. On Tuesday, the progressive strategist Rachel Bitecofer said: “Justice Thomas will get to weigh in on whether Trump engaged in insurrection for the same plot his own wife helped organise. Extraordinary.” Earlier, in a scene of extraordinary Washington pageantry, Biden addressed Thomas and the other justices at a memorial service for Sandra Day O’Connor, the first woman to sit on the court. Speaking at the National Cathedral, the president delivered a passage that would within hours assume greater significance. To O’Connor, Biden said, the court was “the bedrock of America. It was a vital line of defence for the values and the vision of our republic, devoted not to the pursuit of power for power’s sake but to make real the promise of America – the American promise that holds that we’re all created equal and deserve to be treated equally throughout our lives.” Citing that need for equality before the law, some prominent observers said the supreme court should uphold the Colorado ruling. J Michael Luttig, a conservative former judge who testified before the House January 6 committee and has written with the Harvard professor Laurence Tribe on the 14th amendment, called the Colorado ruling “historic”, “masterful” and “brilliant”. “It will be a test of America’s commitment to its democracy, to its constitution and to the rule of law,” Luttig told MSNBC, adding: “Arguably, when it is decided by the supreme court, it will be the single most important constitutional decision in all of our history. “… It is an unassailable … decision that the former president is disqualified from the presidency because he conducted, engaged in or aided or supported an insurrection or rebellion against the United States constitution.” But others were not so supportive. Jonathan Turley, a conservative law professor from George Washington University who has appeared as a witness for House Republicans seeking to impeach Biden on grounds of supposed corruption, told Fox News: “This court has handed partisans on both sides the ultimate tool to try to shortcut elections. And it’s very, very dangerous. “This country is a powder keg, and this court is throwing matches at it. And I think it’s a real mistake. I think they’re wrong on the law. You know, January 6 was many things, most of it not good. In my view it was not an insurrection, it was a riot. “That doesn’t mean the people responsible for that day shouldn’t be held accountable. But to call this an insurrection for the purposes of disqualification would create a slippery slope for every state in the union. “This is a time where we actually need democracy. We need to allow the voters to vote to hear their decision. And the court just said, ‘You’re not going to get that in Colorado, we’re not going to let you vote for Donald Trump.’ You can dislike Trump, you can believe he’s responsible for January 6, but this isn’t the way to do it.” Adopted in 1868, section three of the 14th amendment barred former Confederates from office after the civil war. But it has rarely been used. In Trump’s case, much legal argument has centered on whether the presidency counts as an office, as defined in the text. In Colorado, a lower court found that it did not. The state supreme court found that it did. That argument now goes to the highest court in the land. After the Colorado ruling, many observers also pointed out that Trump has not been convicted of inciting an insurrection, or charged with doing so. He was impeached for inciting an insurrection on January 6 but acquitted at trial in the Senate, where enough Republicans stayed loyal. What is clear is that thanks to Colorado, a US supreme court already racked by politics and with historically low approval ratings will once again pitch into the partisan fight. On Tuesday, Trump seized on the Colorado ruling as he has his criminal indictments: as battle cry and fundraising tool. His Republican opponents also slammed the ruling. Last month, the Pulitzer prize-winning historian Eric Foner, an expert on the civil war and Reconstruction, spoke to the Guardian about 14th amendment challenges to Trump, including in Colorado. A successful case, Foner said, would be likely to act on Trump like “a red flag in front of a bull”. So, it seems clear, will anything the US supreme court now does regarding the Colorado ruling. On Wednesday a Trump attorney, Jay Sekulow, said on his own internet show he expected the court to act quickly, with “the next 10 days … critical in this case” and oral arguments likely by mid-January. His son and co-host, Jordan Sekulow, countered that a slow-moving case could not be counted out.