Trump's eligibility for the ballot is being challenged under the 14th Amendment. Here are the notable cases.

Washington — Decisions in Maine and Colorado finding that former President Donald Trump is disqualified from holding public office have rested on a 155-year-old constitutional provision that was seldom used in modern times, but has now been invoked in escalating legal challenges from across the country targeting Trump's eligibility to hold the presidency again.

The efforts aimed at Trump's ability to appear on ballots under the Constitution's so-called insurrection clause, Section 3 of the 14th Amendment, began just months after the Jan. 6, 2021, attack on the U.S. Capitol. That's when the liberal advocacy group Free Speech for People sent letters to the top election officials in all 50 states urging them to bar the former president from the ballot as a candidate for the White House in 2024.

Since then, legal challenges to Trump's eligibility under the measure have been brought in federal and state courts in more than two dozen states — 26 cases were filed by a little-known Republican presidential candidate named John Anthony Castro. Most cases brought by Castro have been dismissed, either voluntarily or by the courts. In 14 states, there are cases pending, according to Lawfare, a national security website that is tracking the Section 3 cases.