(Sarah Parnass/The Washington Post)

He was interrupted, many times, by strident protesters. He was grilled about the 2016 presidential campaign — everything from cries of “lock her up” to Donald Trump boasting about an unwanted sexual advance, on tape. When one senator asked attorney general nominee Jeff Sessions to explain an old quote about criminal sentencing, the Alabama senator bristled.

“I think that’s rather unfair, based on our relationship and how we work together,” Sessions told Sen. Dick Durbin (D-Ill.).

“Senatorial courtesy,” the unwritten tradition by which a senator nominated for a Cabinet post typically wins easy confirmation, may be the latest norm lost in the tumult of President-elect Donald Trump’s victory. On Monday night, Sen. Cory Booker (D-N.J.) announced plans to become the first senator ever to testify against a colleague. On Tuesday morning, he watched the first few hours of the day of Sessions’s hearings from a seat right behind the committee’s Democrats.

“To remain silent at this time to me is unacceptable,” Booker told The Washington Post. “Even if it means breaking norms on issues of this kind of gravity, I could not have sat well with my own self to remain silent on issues that are the core of our conceptions of justice.”

Inside the hearing room, protests broke out at quiet moments. Several in the audience called Sessions and Trump racists or fascists; one simply laughed loudly when Sessions was praised for his character. Outside the room, Janet Murguia, president of the National Council of La Raza — a Hispanic civil rights group — cheered Booker’s decision to take the unprecedented step of testifying against a fellow senator.

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“I think everything’s fair right now,” she said. “We’re seeing a senator who has said that in some ways he’s changed since what we saw in 1986, but what we have heard and what he has said hasn’t reflected any changes, particularly as it relates to the Hispanic community and immigrants.”

Sessions is not quite the first senator to face stiff opposition from his peers. In 2013, Republicans blistered a former Nebraska senator, Chuck Hagel, on his way to the Pentagon. And in 2001, Democrats nearly blocked the nomination of former Missouri senator John Ashcroft to lead the Justice Department. The last Cabinet nominee to lose a high-profile confirmation battle, in 1989, was former Texas senator John Tower.

Sessions, well-liked personally in the Senate, did not arrive at the hearing under any new cloud of scandal. Instead, Democrats and outside groups had spent the months since his nomination raking over the issues that had torpedoed his 1986 bid for a judgeship and his senatorial views of the Department of Justice’s civil rights enforcement. On Tuesday, a letter surfaced from the widow of civil rights leader Martin Luther King Jr., who had urged Congress to block Sessions’s nomination for the judiciary.

Senate Democrats, often prefacing their remarks by describing their friendship with the senator, went on to see where Sessions would back away from his record or from Trump.

Sen. Al Franken (D-Minn.), who said that Democrats would not misrepresent Sessions — “certainly don’t want to do that to our colleague” — used much of his time to establish that Sessions’s defenders had overstated his role in prosecuting some voter discrimination cases. Sen. Christopher A. Coons (D-Del.) asked Sessions repeatedly to reject the idea of a registry or database for Muslims in America.

“This experience is a difficult one, not only because you’re a colleague,” said Sen. Richard Blumenthal (D-Conn.) “I consider you to be a friend.” From there, he asked Sessions if he would appoint special counsels to investigate possible Trump conflicts of interest, and got inconclusive answers.

But Booker’s plans, which first surfaced Monday, represented the highest-profile challenge to Sessions. On Monday night, Republicans quickly began circulating clips of the New Jersey senator praising Sessions at a ceremony last year. The two had worked with other lawmakers to ensure that activists who’d marched for voting rights earned the Congressional Gold Medal.

“I feel blessed and honored to have partnered with Sen. Sessions in being the Senate sponsors of this important award,” Booker said at the event last February.

In an interview, Booker said that he would address his past praise of Sessions in his testimony.

“I am really proud to have worked with Jeff Sessions on awarding that medal to those marchers,” he said. “I am really grateful for the collegial relationship that he and I have had, the frank conversation, the decorum with which he had greeted me and I hope that he thinks I have greeted him. I met with him [Monday] with staff there, and that feeling of goodwill was there and he knew I was going to testify.”

Current and former senators expressed surprise at Booker’s decision, and at the work Democrats and allied groups were doing to weigh down Sessions with his voting record and quotes.

“It does show the partisan divide in the Senate today,” said former Republican senator Mike DeWine, who is now Ohio’s attorney general. “That was starting when I was here, we had certainly some of that. It appears to me to have gotten worse. I think it’s sad.”

DeWine added that he thought Booker’s testimony would have “no impact” on Sessions’s ultimate confirmation.

So did Sen. Jeff Flake (R-Ariz.), who said that Sessions’s surprise announcement that he plans to recuse himself from any Justice Department dealings with former secretary of state Hillary Clinton and her ties to her family’s charitable foundation, “ought to give those who had an issue with him some comfort.”

Flake had arrived in Washington as a congressman in 2001; DeWine reached the Senate in 1995. Sen. Tom Cotton (R-Ark.) pronounced himself “very disappointed that Senator Booker has chosen to start his 2020 presidential campaign” with the testimony.

“Sen. Booker says he feels compelled to speak out because Sen. Sessions wants to keep criminals behind bars, drugs off our streets, and amnesty from becoming law,” Cotton wrote. “Sen. Booker is better than that and he knows better.”

Sen. Tim Scott (R-S.C.), the chamber’s only black Republican senator, announced late Monday that he would be voting to confirm Sessions. He did so after months of consideration that included research of his colleague’s past statements and a visit with Sessions to meet with law enforcement and religious leaders in Charleston, S.C.

“I thought it was my responsibility to be painstaking on understanding and appreciating what has been said and what is true” about Sessions, Scott explained. “And then, do enough research to come to a conclusion based on people who know him intimately and what they say as well.”

Scott said he spoke with Albert Turner Jr., whose parents were prosecuted by Sessions for voter fraud — an incident that senators cited as a reason to reject Sessions for a federal judgeship in 1986 – and who has endorsed Sessions for attorney general. Scott said he also spoke with other Alabama Democrats and civil rights leaders who were supporting Sessions despite their decades of work that might seem counter to the Alabama senator’s positions.

After that careful study, “I think I have a pretty clear picture of who he is, and with that information it was easy for me to conclude that the reality of Jeff Sessions is different than what people are protesting today,” Scott said. “I like Cory Booker, and I think it’s an unusual, obviously unique — first time I think it’s been done — I think we’ll know what this means 20 years looking back.”

Booker is set to testify alongside Reps. John Lewis (D-Ga.), a civil rights icon who marched with King through Selma, Ala., in 1965, and Cedric L. Richmond (D-La.), the new chairman of the Congressional Black Caucus, a group has promised to be a forceful critic of the Trump administration in the coming years.