A trial to determine former Pres. Trump's eligibility to appear on the 2024 ballot is officially underway in Colorado.

What's happening? A Colorado court on Mon. heard opening remarks in a case seeking to block Trump from the 2024 ballot.

The lawsuit was filed by six Colorado voters in Sept. with Citizens for Responsibility and Ethics in Washington. (NYT)

What is their argument? The lawsuit argues that Trump is ineligible to serve under Section 3 of the 14th Amendment. (NYT)

Section 3 of the 14th Amendment bars U.S. officials who "engaged in insurrection or rebellion" from running for office.

Section 3 also disqualifies U.S. officials who have "given aid or comfort" to insurrectionists. (CNN)

According to CNN, Section 3 has only been used twice since its ratification, primarily against former Confederates.

Attorney Eric Olson argued Mon. that Trump's actions on Jan. 6 met the Constitution's bar for disqualification.

Olson: "...our Constitution, the shared charter of our nation, says he cannot [run for president again.]"

Colorado's trial is one of two cases seeking to test the 14th Amendment's "insurrectionist ban." (AP)

Minnesota's Supreme Court is also set to hear arguments in a similar case on Thu.. Both cases could land before SCOTUS.