It was a leak clearly designed to make William Barr's day on Capitol Hill far more unpleasant.

The source or sources who showed The Washington Post a letter of complaint that Bob Mueller had written Barr created a media explosion that reverberated all day yesterday, when the attorney general had been slated to testify before a Senate committee. Even before he took the hot seat, some Democrats were calling on Barr to resign — which has virtually no chance of happening.

Once the Judiciary Committee hearing got underway, it was so utterly partisan that it seemed Republicans and Democrats were operating in parallel universes — and that tended to muffle the uproar over the once-secret Mueller letter.

Still, the letter hurts Barr's reputation, no question about it. The missive provides ammunition to the AG's critics, who say he acted like a Trump partisan in spinning and perhaps minimizing the Mueller report's findings.

But let's face it: the special counsel's letter would have been far more damaging had it emerged before the report was made public, when the debate over Barr's conduct was at its peak. Now that we've all had the 448-page report for a couple of weeks, this has the feel of relitigating a process question that's been overtaken by events.

"The letter and a subsequent phone call between the two men reveal the degree to which the longtime colleagues and friends disagreed as they handled the legally and politically fraught task of investigating the president," the Post says.

The paper quotes the Mueller note as dissing Barr's famous four-page summary before the report was out:

"The summary letter the Department sent to Congress and released to the public late in the afternoon of March 24 did not fully capture the context, nature, and substance of this office's work and conclusions. There is now public confusion about critical aspects of the results of our investigation. This threatens to undermine a central purpose for which the Department appointed the Special Counsel: to assure full public confidence in the outcome of the investigations."

Mueller asked that his own executive summaries be quickly released, but Barr declined.

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One reason the leaked letter landed with considerable force is that we never hear Mueller express opinions in his own voice, rather than in legal filings or the rare statements from his office. He is the offstage presence, the opposite of a grandstander, even with the report having been made published. The public will finally hear Mueller speak in House testimony this month, according to an agreement announced yesterday.

But clearly one of his allies — whether it was with Mueller's acquiescence or not, we don't know — wanted to turn up the heat before Barr's testimony.

The GOP side, led by Lindsey Graham, mainly wanted to talk about Hillary Clinton's emails and Trump-sliming emails from the FBI's Peter Strzok and Lisa Page (complete with an F-word that the senator read on live television). The Democratic side, led by Dianne Feinstein, read damaging passages from the report and pressed Barr about his disagreements with Mueller and why he didn't see many of the findings as obstruction of justice.

What was most noteworthy was Barr admitting he was surprised when Mueller declined to reach a conclusion on obstruction allegations and saying he could not get a clear explanation while meeting with him. The implication was that Mueller, given his independence, should have made the call, and instead made the report what Barr called "my baby."

The attorney general insisted that Mueller "was very clear with me that he was not suggesting that we had misrepresented his report." In a shot at the media, Barr said Mueller told him that "the press reporting had been inaccurate and that the press was reading too much into it."

Oddly enough, Barr also said Mueller declined his offer to review the four-page summary in advance.

Feinstein pressed the AG about the finding that Trump told his White House counsel, Don McGahn, to have Mueller fired, and that McGahn refused and threatened to resign.

This was not an attempt to obstruct the probe, Barr said, because "there is a distinction between saying to someone, 'Go fire him, go fire Mueller,' and saying, 'Have him removed based on conflict.'" But there was no obstruction, Barr said, because "presumably" someone else would have been named to replace Mueller. (McGahn regarded the conflict questions as "silly.")

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Things turned absurdly partisan when Sen. Mazie Hirono demanded that Barr resign, saying he had sacrificed his "once-decent reputation for the grifter and liar who sits in the Oval Office." Graham shot back, "Listen, you slandered this man!" And the three presidential candidates on the panel — Kamala Harris, Amy Klobuchar and Cory Booker — all got their licks in. Harris, Booker and former Vice President Joe Biden also demanded the AG's resignation.

In the end, the spat between Barr and Mueller will be a historical footnote. But it provides more fodder for the Democrats and Trump's media critics to try to keep the investigation alive.