Fox News senior legal analyst Judge Andrew Napolitano appeared on "The Story with Martha MacCallum" Monday to discuss how testimony from former National Security Adviser John Bolton could impact the Senate impeachment trial.

"If he does testify, he opens up the floodgates to a real trial, meaning, as Senator [Chuck] Schumer [D-N.Y.] has been saying as recently as a few hours ago, live testimony and documents, not summaries of what people told the House Judiciary or House Intelligence Committee as they did in the in the Clinton investigation," Napolitano said. "That's what's good news for the Democrats and bad news for the president."

BOLTON, MULVANEY INCLUDED IN SENATE DEMS' WISH LIST FOR IMPEACHMENT TRIAL

Bolton said Monday that he would testify should he be subpoenaed by the Senate.

Bolton has been sought by Schumer and other Senate Democrats as a witness along with three other administration officials, including acting White House Chief of Staff Mick Mulvaney. But the Republican majority has not agreed to witnesses, and the entire process is in a holding pattern as House Speaker Nancy Pelosi, D-Calif., sits on the impeachment articles.

"There's another side to this, and that is we don't know what John Bolton will say and he might not give an interview," Napolitano added. "Now, it would be crazy for a prosecutor to call a witness to testify without knowing ahead of time what the witness is going to say in the real world, and impeachment, [while] it's not quite the real world, that would be malpractice whatever the outcome."

MacCallum raised the possibility that Bolton could say the president shouldn't be impeached if he is called to testify.

CLICK HERE TO GET THE FOX NEWS APP

"Or he could say, 'I was there when the president said ... 'I'm not giving this guy [Ukraine President Volodymyr Zelensky] his money until he goes after Joe Biden,'" Napolitano responded. "I mean, but [if] something like that comes out of his mouth that would be very hard for the president. And it might force the president to take the stand in his own defense."

Napolitano also believes that Senate Democrats will convince four Republicans to agree to hear Bolton as a witness due to fears among moderate GOP senators of a backlash from the media and members of the public.

Fox News' Brooke Singman contributed to this report.