DENVER (AP) — Then-President Donald Trump could have mobilized the National Guard and other federal agencies to protect the U.S. Capitol once violence broke out on Jan. 6, 2021, a law professor testified Tuesday as a case to bar the former president from the 2024 ballot moved into a new phase.

William Banks, a Syracuse University law professor and expert in national security law, said that once the attack on the Capitol began, Trump had options he did not use.

"He should respond to his constitutional responsibilities to protect the security of the United States when there's an assault on our democratic process," Banks said of Trump.

Banks was testifying as a witness for a group of Colorado voters who want to bar Trump from the ballot for allegedly violating his oath to uphold the Constitution in his attempt to overturn his 2020 election loss. They cite a rarely used clause in the 14th Amendment, adopted just after the Civil War, that prohibits those who "engaged in insurrection" against the Constitution from holding higher office.

The Colorado case and another before the Minnesota Supreme Court on Thursday are the two most advanced of dozens of challenges to Trump around the country. On Tuesday, his campaign filed a lawsuit in Michigan to stop a related case there. One of the cases is likely to end up at the U.S. Supreme Court, which has never ruled on the insurrection clause.

Trump's attorneys have said the lawsuits are anti-democratic efforts to deny voters the opportunity to choose who leads the country. They've also said they will show in court that Trump took the threat of violence seriously on Jan. 6.

On Tuesday, Geoffrey Blue, one of Trump's Colorado attorneys, noted that a Department of Defense timeline shows that Trump previously had asked Gen. Mark Milley, then-head of the Joint Chiefs of Staff, if the military was prepared for protests that he himself had called against Congress' Jan 6 certification of President Joe Biden's win. Milley said yes, according to the timeline.

Trump gave a fiery speech on the Ellipse before the assault, urging his supporters to march to the Capitol to protest the fictitious election fraud he claimed had stopped him from winning a second term. Attacks on the Capitol began earlier and intensified significantly after Trump wrapped up his speech.

Banks acknowledged he did not know what conversations were occurring in the White House that afternoon. Testimony to the House committee that investigated the attack shows that some aides were aghast that Trump wasn't taking action, but the former president's lawyers called the committee's report a partisan document that was distrusted by Republicans.

The committee's final report details Trump's inaction as rioters stormed the Capitol, calling it "a dereliction of duty," and suggested that Congress consider barring him from holding office in the future.

Earlier Tuesday, an expert in right-wing extremism testified about how violent groups interpreted Trump's calls for a "wild" protest on Jan. 6. And on Monday, the trial's opening day, police officers who defended the Capitol that day testified about the assault.

The case is expected to get to the constitutional issues that make it significant on Wednesday, when a law professor is scheduled to testify about whether Section Three of the 14th Amendment applies to the president, even though that office is not specifically mentioned in the text.

Trump's attorneys and some critics of the litigation contend the provision also needs congressional legislation to be used.