Deeply serious questions about the abuse of power and intentional civil liberties violations are emerging as the American public learns more details about the FBI’s investigation of President Donald Trump.

But don’t take the word of the president’s defenders. Listen to a liberal historian who donated to the presidential campaign of Vermont socialist Sen. Bernie Sanders and voted for Democrat Hillary Clinton in the general election.

“To me, as a liberal Democrat, it’s really offensive to see Democratic politicians sticking up for and defending and excusing the FBI’s behavior, just because their opposition to President Trump is so intense that it’s leading them to abandon what previously have been democratic civil liberties principles,” David Garrow said Wednesday on “The Laura Ingraham Show.”

Garrow, who won the Pulitzer Prize in 1986 for his book “Bearing the Cross: Martin Luther King, Jr., and the Southern Christian Leadership Conference,” noted that the FBI and the CIA have a long history of overreach that went unchecked for decades.

These were things liberals were concerned about at one time.

“They really weren’t constrained at all, either by presidents or by anyone in Congress,” he said. “You know, J. Edgar Hoover [the longtime FBI director] did a great job of getting both Franklin Roosevelt and Lyndon Johnson to be, you know, very strong supporters of him. And he had great friendly supporters in the Congress, too.”

The political branches pushed back against the FBI beginning in the 1970s, Garrow said. He argued that progressives should be leery of the secretive Foreign Intelligence Surveillance Act (FISA), which allows judges to give permission to FBI agents to conduct surveillance of Americans as part of counterintelligence investigations.

The FBI, with the approval of the Department of Justice, used that tool to monitor Trump campaign volunteer Carter Page — including reading his email, listening to his telephone conversations, bugging his home and office, and recording his movements — even though it now appears that the basis for that surveillance in large part was opposition research financed by Clinton and the Democratic National Committee.

“The whole process that allows for domestic electronic surveillance through the Foreign Intelligence Surveillance Court — that’s not only a very opaque, top-secret process, but it certainly seems, you know, this last year or so, that it’s been abused for partisan purposes,” Garrow said.

Garrow criticized the document produced by that opposition research — a dossier prepared by former British spy Christopher Steele. It first became public last year when BuzzFeed published it in its entirety, without prior verification.

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“It is 100 percent, complete C-R-A-P,” he said. “Anyone who’s read intelligence documents for decades and decades like I have, thanks to the Freedom of Information Act, could immediately recognize that what Steele was peddling was third-, fourth-hand chatter from, you know, anonymous, unnamed people who would talk to other anonymous, unnamed people. It struck me as entirely unreliable, and I’m no fan of Donald Trump.”

Garrow also responded to new reports that FBI employees Peter Strzok and Lisa Page discussed then-President Barack Obama’s desire to “know everything” they were doing as part of the investigation into Clinton’s handling of classified information as secretary of state.

Related: Nunes Says Obama State Dept. Passed Trump Dirt ‘to Very Strange Places’

That contradicts Obama’s pronouncement at the time that he never talked to law enforcement authorities about pending investigations.

“As someone who’s been a historian for 40 years, I know very well that contemporaneous documents are the best possible sources,” said Garrow. “Now maybe, you know, the agent got something wrong, but he was, as you mentioned, right at the center of this entire thing.”

In evaluating the truth, Garrow said, “That would be very, very important.”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.