Washington (CNN) Former Attorney General Eric Holder believes William Barr is "unfit" to serve as the nation's top law enforcement officer, writing in a Washington Post op-ed that recent controversial comments by his successor "have been fundamentally inconsistent with his duty to the Constitution."

In the opinion piece published Wednesday night, Holder, who served in the Obama administration between 2009 and 2015, excoriated Barr for "a series of public statements and ... actions that are so plainly ideological, so nakedly partisan and so deeply inappropriate" for an attorney general to make. He pointed to a number of recent headline-grabbing remarks by Barr, including his comments this week that the FBI may have acted in "bad faith" when it opened an investigation into the origins of the Russia probe despite the Justice Department's inspector general report that said the bureau was justified in doing so.

"(It) was infuriating to watch him publicly undermine an independent inspector general report -- based on an exhaustive review of the FBI's conduct -- using partisan talking points bearing no resemblance to the facts his own department has uncovered," he wrote.

Holder, who said he was "reluctant to publicly criticize (his) successors," also blasted Barr for comments he made last month at an event held by the Federalist Society, a conservative legal group, in which the attorney general affirmed his credentials as a staunch defender of presidential power, trashed the so-called resistance movement and mocked Senate Democrats.

"To me, his attempts to vilify the President's critics sounded more like the tactics of an unscrupulous criminal defense lawyer than a U.S. attorney general," Holder wrote, adding that in his comments, Barr "exposed himself as a partisan actor, not an impartial law enforcement official."

Read More