While Democrats tapped Gerhardt for Wednesday’s hearing, his expertise in the past has been sought by both sides of the aisle. He’s testified multiple times via invitations from both Republicans and Democrats who wanted to hear him discuss his books and other published research, including a conclusion he reached in 1996 that impeachable offenses weren’t limited to just criminal acts.

Gerhardt has remained a go-to voice on impeachment during the Trump era. In September, he said the president’s legal scattershot strategy was in part “to just produce confusion and chaos.”

“That’s served him well until now,” he told POLITICO. “It’s his way of blowing smoke. And so having different people speaking for him in different ways and at different times, and his own bluster on Twitter and elsewhere, all that together is his defense. It’s designed to keep people off balance.”

Turley is also no stranger to congressional impeachment fights. In 2010, he served as lead defense counsel in the Senate impeachment trial that ended in the removal of U.S. District Judge G. Thomas Porteous, an appointee of President Bill Clinton convicted on four articles tied to charges including corruption. The lead House impeachment manager in those proceedings: Rep. Adam Schiff.

In September, former Trump White House attorney Ty Cobb floated in an interview with POLITICO both Turley and Alan Dershowitz as potential hires for the president should he opt to mount a more forceful impeachment defense.

Feldman, who is a contributing writer to Bloomberg Opinion, has also weighed in on Democrats‘ case for impeachment quite regularly, including an Oct. 20 piece titled “Trump’s Quid Pro Quo Is Unconstitutional.“

Also Monday, Trump’s personal legal team mocked the Democrats for turning to constitutional experts rather than first-hand witnesses who could identify impeachment-worthy behavior by the president.

“There’s nothing the American people want to hear less than a bunch of overly educated law professors give their advice” about impeachment, Jordan Sekulow, an attorney for the president, said on his father’s talk radio show.

Appearing before the House panel named its witnesses, Jordan Sekulow said some of the president’s personal lawyers planned to dig into the published work of the professors in search of controversy.

“I can’t wait to find out what crazy stuff the law professors have written,” Jordan Sekulow said. “I bet anti-Israel, borderline anti-Semitic. Maybe anti-American?”