Maybe we now know the real reason that Bob Mueller was so reluctant to testify.

With even liberal commentators conceding that Mueller was a shaky witness during two House hearings, questions are swirling about his mental acuity and his ability to handle the job of special counsel.

Let me say at the outset that I have great respect for Mueller as a decorated Vietnam War veteran and an FBI director so widely admired that Barack Obama asked the George W. Bush appointee to serve a second term.

Let me also say that the hearings shouldn’t be graded only on optics, although they were, like most hearings, designed as political theater. But even on substance, Mueller offered almost nothing that was new, and for all the media hype, that was very much by design.

Still, Mueller’s struggles on the Hill were a real head-scratcher, especially for those who have worked with him.

The New York Times reported on the front page yesterday that, as the prosecutor in charge of a two-year investigation of President Trump and Russian interference, he was not the Mueller of old:

“Soon after the special counsel’s office opened in 2017, some aides noticed that Robert S. Mueller III kept noticeably shorter hours than he had as F.B.I. director, when he showed up at the bureau daily at 6 a.m. and often worked nights.

He seemed to cede substantial responsibility to his top deputies, including Aaron Zebley, who managed day-to-day operations and often reported on the investigation’s progress up the chain in the Justice Department. As negotiations with President Trump’s lawyers about interviewing him dragged on, for example, Mr. Mueller took part less and less, according to people familiar with how the office worked.

That hands-off style was on display on Wednesday when Mr. Mueller testified for about seven hours before two House committees. Once famous for his laserlike focus, Mr. Mueller, who will turn 75 next month, seemed hesitant about the facts in his own 448-page report. He struggled at one point to come up with the word ‘conspiracy.’”

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Mueller, who asked for questions to be repeated more than a dozen times, even botched one about which president appointed him as a top prosecutor in 1986.

So if Times reporters (and presumably other reporters) knew that Mueller was a hands-off leader with dwindling stamina who increasingly relied on his deputies, how did that remain such a closely guarded secret?

I don’t want to cast aspersions on journalists who have doggedly covered the investigation, but the temptation not to jeopardize their access, and the possibility of future leaks, must have been considerable. Now that Mueller is no longer special counsel, and his shortcomings were so glaringly on public display, it’s “safe” to publish the story.

David Axelrod, who knows him from his time in the Obama White House, tweeted: “This is delicate to say, but Mueller, whom I deeply respect, has not publicly testified before Congress in at least six years. And he does not appear as sharp as he was then.”

Now I don’t think it’s fair to expect Mueller to know every detail of a sprawling investigation or every sentence in the report. He was under tremendous pressure not to get anything wrong, and self-imposed pressure not to break any new ground.

And I don’t think it’s fair for commentators to speculate or insinuate that he might have some kind of medical condition.

But in describing his “painful” testimony, the Times said Mueller’s “halting delivery stood out all the more given his towering reputation for a command of facts and physical stamina — the stuff of lore among his former aides and colleagues. Nonetheless, he was unmistakably shaky.”

And the paper reported that calendars show one of the top prosecutors, Andrew Weissman, met infrequently with Mueller, except for a daily 5 p.m. staff meeting. But the calendars say his aide Zebley was the team leader at these meetings 111 times.

As for the fallout, the Washington Post’s Dan Balz said Mueller was supposed to be the Democrats’ savior but the hearings “probably shattered those illusions once and for all. If Democrats hope to end the Trump presidency, they will have to do so by defeating him at the ballot box in November 2020.”

Some liberals, such as pro-impeachment Harvard Law professor Laurence Tribe, were candid about what happened. He said the hearings were “a disaster. Far from breathing life into his damning report, the tired Robert Mueller sucked the life out of it.”

But some MSNBC opinion hosts seized on a few words here or there, as if Mueller hadn’t said in his report four months ago that the report didn’t “exonerate” Trump.

When Mueller told House Intel chairman Adam Schiff that knowingly accepting foreign help in a presidential campaign is “a crime in certain circumstances,” that’s hardly the same as saying the Trump team was guilty of such a crime, which his report did not find.

Another sound bite popular at MSNBC was this brief exchange with Democratic Rep. Ted Lieu, who said the reason "you did not indict Donald Trump is because of the OLC opinion stating that you cannot indict a sitting president, correct?”

“That is correct,” Mueller said.

Despite the fact that Mueller started the second hearing by saying he had to “correct” something—“We did not reach a determination as to whether the president committed a crime”—some at the cable network seemed to place more weight on the first answer.

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What the bobbled response also showed was a witness who was not quite up to the task, something we’re now learning was an open secret in at least some Washington circles.