Mr. Barr tolerantly sat through hours of Democratic insults at a Senate Judiciary Committee hearing Wednesday. His reward for his patience was to be labeled, in the space of a news cycle, a lawbreaking, dishonest, obstructing hack. Speaker Nancy Pelosi publicly accused Mr. Barr of lying to Congress, which, she added, is “considered a crime.” House Judiciary Committee Chairman Jerrold Nadler said he will move to hold Mr. Barr in contempt unless the attorney general acquiesces to the unprecedented demand that he submit to cross-examination by committee staff attorneys. James Comey, former director of the Federal Bureau of Investigation, lamented that Donald Trump had “eaten” Mr. Barr’s “soul.” Massachusetts Sen. Elizabeth Warren demands the attorney general resign. California Rep. Eric Swalwell wants him impeached.

These attacks aren’t about special counsel Robert Mueller, his report or even the surreal debate over Mr. Barr’s first letter describing the report. The attorney general delivered the transparency Democrats demanded: He quickly released a lightly redacted report, which portrayed the president in a negative light. What do Democrats have to object to?

Some of this is frustration. Democrats foolishly invested two years of political capital in the idea that Mr. Mueller would prove President Trump had colluded with Russia, and Mr. Mueller left them empty-handed. Some of it is personal. Democrats resent that Mr. Barr won’t cower or apologize for doing his job. Some is bitterness that Mr. Barr is performing like a real attorney general, making the call against obstruction-of-justice charges rather than sitting back and letting Democrats have their fun with Mr. Mueller’s obstruction innuendo.

But most of it is likely fear. Mr. Barr made real news in that Senate hearing, and while the press didn’t notice, Democrats did. The attorney general said he’d already assigned people at the Justice Department to assist his investigation of the origins of the Trump-Russia probe. He said his review would be far-reaching—that he was obtaining details from congressional investigations, from the ongoing probe by the department’s inspector general, Michael Horowitz, and even from Mr. Mueller’s work. Mr. Barr said the investigation wouldn’t focus only on the fall 2016 justifications for secret surveillance warrants against Trump team members but would go back months earlier.

He also said he’d focus on the infamous “dossier” concocted by opposition-research firm Fusion GPS and British former spy Christopher Steele, on which the FBI relied so heavily in its probe. Mr. Barr acknowledged his concern that the dossier itself could be Russian disinformation, a possibility he described as not “entirely speculative.” He also revealed that the department has “multiple criminal leak investigations under way” into the disclosure of classified details about the Trump-Russia investigation.

Do not underestimate how many powerful people in Washington have something to lose from Mr. Barr’s probe. Among them: Former and current leaders of the law-enforcement and intelligence communities. The Democratic Party pooh-bahs who paid a foreign national (Mr. Steele) to collect information from Russians and deliver it to the FBI. The government officials who misused their positions to target a presidential campaign. The leakers. The media. More than reputations are at risk. Revelations could lead to lawsuits, formal disciplinary actions, lost jobs, even criminal prosecution.

The attacks on Mr. Barr are first and foremost an effort to force him out, to prevent this information from coming to light until Democrats can retake the White House in 2020. As a fallback, the coordinated campaign works as a pre-emptive smear, diminishing the credibility of his ultimate findings by priming the public to view him as a partisan.

That’s why Mr. Barr isn’t alone in getting slimed. Natasha Bertrand at Politico last month penned a hit piece on the respected Mr. Horowitz. It’s clear the inspector general is asking the right questions. The Politico article acknowledges he’s homing in on Mr. Steele’s “credibility” and the dossier’s “veracity”—then goes on to provide a defense of Mr. Steele and his dossier, while quoting unnamed sources who deride the “quality” of the Horowitz probe, and (hilariously) claim the long-tenured inspector general is not “well-versed” in core Justice Department functions.

“We have to stop using the criminal-justice process as a political weapon,” Mr. Barr said Wednesday. The line didn’t get much notice, but that worthy goal increasingly looks to be a reason Mr. Barr accepted this unpleasant job. Stopping this abuse requires understanding how it started. The liberal establishment, including journalists friendly with it, doesn’t want that to happen, and so has made it a mission to destroy Mr. Barr. The attorney general seems to know what he’s up against, and remains undeterred. That’s the sort of steely will necessary to right the ship at the Justice Department and the FBI.