Retired justice John Paul Stevens didn’t like what he saw. Neither did the American Bar Association panel that first gave U.S. Appeals Court Judge Brett M. Kavanaugh its highest endorsement for his elevation to the Supreme Court — and Friday announced that it was having second thoughts.

And Senate Democrats say — putting aside other allegations and concerns about Kavanaugh moving the Supreme Court too far to the right — the judge’s angry and confrontational testimony before the Senate Judiciary Committee last week undermines his fitness for the court.

As Kavanaugh’s confirmation seemed all but guaranteed Friday, questions lingered about his judicial temperament, a difficult-to-define term that took on enormous significance in the bruising battle over his nomination.

The concept of judicial temperament is considered important to assuring the public that a judge is a neutral arbiter, someone with an open mind and without an agenda. Nonpartisanship would seem a necessary part of that, along with, on an appellate court like the Supreme Court, collegiality and negotiating skills that persuade others to form a majority for an outcome.

But if judicial temperament is enough to persuade some to support or oppose a judge, its exact quality is hard to define.

“Judicial temperament is simultaneously the thing we think all judges must have and the one thing that no one can quite put a finger on,” Vanderbilt law professor Terry A. Maroney writes in a forthcoming paper on the subject.

“Being perceived as having a good temperament can get a judge confirmed or elevated; the opposite can stop her at the gate, or, if she already is serving, get her reprimanded or removed from the bench.”

In other words, people seem to think they know it when they see it, and those evaluating Kavanaugh say it was missing in his appearance Sept. 27, when he responded to allegations of a teenage sexual assault and excessive drinking.

“We saw a man filled with anger and aggression,” Sen. Dianne Feinstein (D-Calif.) said in explaining on the Senate floor Friday why she opposed Kavanaugh.

“Judge Kavanaugh raised his voice, he interrupted senators. He accused Democrats of ‘lying in wait’ and replacing ‘advice and consent with search and destroy.’ . . . This behavior revealed a hostility and belligerence that was unbecoming of someone seeking to be elevated to the United States Supreme Court.”

His supporters ask: What else could be expected of a man whose career was on the line? “Judge Kavanaugh was publicly accused of a crime, and his reputation and livelihood were at stake,” Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) said Friday.

But Kavanaugh himself seemed to recognize that the display in the Senate could undermine his 12 years as a judge on the U.S. Court of Appeals for the District of Columbia Circuit. He took the highly unusual move of writing an op-ed in the Wall Street Journal that was part defense, part apology.

“I know that my tone was sharp, and I said a few things I should not have said,” Kavanaugh wrote. “I hope everyone can understand that I was there as a son, husband and dad. I testified with five people foremost in my mind: my mom, my dad, my wife, and most of all my daughters.”

He pledged it won’t happen again: “Going forward, you can count on me to be the same kind of judge and person I have been for my entire 28-year legal career: hardworking, even-keeled, open-minded, independent and dedicated to the Constitution and the public good.”

Maroney said being nonpartisan is related to but apart from her definition of judicial temperament.

But it seemed to be what made Stevens change his mind about Kavanaugh.

At an event before retirees in Boca Raton, Fla., on Thursday, Stevens said he thought enough of the judge’s opinions to feature him in a book. But he said he agreed with some critics that some of Kavanaugh’s comments at the hearing were so sharp that they might warrant recusal if certain groups — Stevens did not specify which ones — were before the Supreme Court.

“They suggest that he has demonstrated a potential bias involving enough potential litigants before the court that he would not be able to perform his full responsibilities,” Stevens said in remarks at the event. “And I think there is merit in that criticism and that the senators should really pay attention to it.”

“For the good of the court,” he said, “it’s not healthy to get a new justice that can only do a part-time job.”

Others said Kavanaugh’s comments were more than spur-of-the-moment outrage.

“Rather than strive, as most others have done, to dissipate the partisan atmosphere, Judge Kavanaugh chose to throw the testimonial equivalent of gasoline on it,” Harvard law professor Richard Lazarus, formerly head of the Georgetown Law Center’s Supreme Court Institute, wrote in an email.

“Perhaps out of uncontrolled fury at the nature of the accusations directed to him. Or perhaps based on a political calculation that it was the best or even only way to garner the support, including that of the tainted president who nominated him, necessary to maximize his prospects of confirmation. I do not know which it was.”

Benjamin Wittes, a senior fellow at the Brookings Institution and someone who said he has long admired Kavanaugh, said the view he got from the hearing partly convinced him that the judge should not be confirmed.

Earlier, Kavanaugh described a good judge as “an umpire — a neutral and impartial arbiter who favors no litigant or policy,” Wittes wrote in the Atlantic, adding, “A very different Brett Kavanaugh showed up to Thursday’s hearing.”

Kavanaugh’s warnings that the charges against him were nurtured by Democrats unable to get past President Trump’s election, and that “what goes around comes around,” were too much, Wittes wrote. “Kavanaugh blew across lines that I believe a justice still needs to hold.”

Sen. Joe Manchin III (W.Va.), the only Democrat who has pledged to vote for Kavanaugh, was more forgiving.

“It was a wild and woolly hearing last week,” Manchin told reporters, in which Kavanaugh was accused of terrible things.

What Manchin saw was a “a father of two young girls fighting back, saying, ‘That’s not the dad I am, that’s not the person I am.’ ”

Some legal academics agreed with Manchin.

Eugene Volokh, a UCLA law professor who runs a legal blog called the Volokh Conspiracy, said Kavanaugh’s reaction was understandable and should be put in context with the judge’s reputation.

“I can’t imagine how I would keep my composure in such a situation, even if I (like Judge Kavanaugh) were a judge who had a decade-long reputation of calm and politeness during the ordinary work (including the controversial work) of a court,” Volokh wrote.

But Volokh was aware that many other law professors disagreed.

More than 2,400 of them, from across the nation, signed an open letter to the Senate saying that they had “united, as professors of law and scholars of judicial institutions, in believing that Judge Kavanaugh did not display the impartiality and judicial temperament requisite to sit on the highest court of our land.”

Mike DeBonis contributed to this report.