A Washington-based ethics watchdog filed a lawsuit on Wednesday to try to block Donald Trump from appearing on the ballot in Colorado next year if he wins the Republican presidential nomination, arguing that his actions on Jan. 6, 2021, disqualify him from office.

The nonpartisan group, Citizens for Responsibility and Ethics in Washington, filed the lawsuit in a Colorado state court on behalf of six Republican and unaffiliated voters including former state, federal, and local officials, it said in a statement.

The untested legal strategy, which relies on a reading of the 14th Amendment to the U.S. Constitution, faces long odds, according to legal experts. It would require legal challenges in all U.S. states and territories to convince officials that the former president is ineligible to serve after his supporters attacked the Capitol on Jan. 6, 2021, in an attempt to overturn his election defeat.

Trump, the front-runner in the race for the Republican nomination to challenge Democratic President Joe Biden in 2024, continues to falsely claim his defeat was the result of fraud.

"While it is unprecedented to bring this type of case against a former president, January 6th was an unprecedented attack that is exactly the kind of event the framers of the 14th Amendment wanted to build protections in case of," Noah Bookbinder, president of the ethics group known as CREW, said in a statement.

The lawsuit cites Section 3 of the 14th Amendment, which bars any person from holding federal or state office who took an "oath ... to support the Constitution of the United States" and then "engaged in insurrection or rebellion against the same," according to the statement.

The watchdog alleges that Trump violated that oath by "recruiting, inciting and encouraging a violent mob that attacked the Capitol on Jan. 6, 2021 in a futile attempt to remain in office."

Trump has pleaded not guilty to charges brought by Special Counsel Jack Smith in federal court in Washington that he conspired to defraud the United States by preventing Congress from certifying Biden's 2020 election victory and to deprive voters of their right to a fair election.

While the lawsuit was filed in Colorado, election analysts rate that state as solidly Democratic and one that Trump would be unlikely to win in 2024 even if he were on the ballot.

(Reporting by Ismail Shakil in Ottawa and Jack Queen in New York; Editing by Scott Malone and Bill Berkrot)