WASHINGTON - Sen. Cory A. Booker (D., N.J.) has cultivated an image as a voice for minorities and as a post-partisan lawmaker trying to bridge divides.
"I'm disappointed," said Sen. Orrin Hatch (R., Utah), who has spent nearly 40 years in the chamber. "It's a fellow senator - you don't treat other senators that way."
Sen. Tom Cotton (R., Ark.), called the move "a disgraceful breach of custom."
"This hearing simply offers a platform for his presidential aspirations. Senator Booker is better than that, and he knows better," Cotton wrote on Facebook.
Booker, who announced the decision Monday, is scheduled to testify Wednesday, when Sessions' confirmation hearing resumes for a second day before the Senate Judiciary Committee. Booker's office said no senator had ever testified against another up for a cabinet post. But he called his decision "a call to conscience" based on the nominee's "deeply troubling" views.
"I do not take lightly the decision to testify against a Senate colleague," he said in a statement Monday night. "The Attorney General is responsible for ensuring the fair administration of justice, and based on his record, I lack confidence that Senator Sessions can honor this duty."
Booker is one of 15 witnesses slated to argue for and against Sessions. Testifying alongside him will be Rep. John Lewis (D., Ga.), a civil rights icon - putting both in the national spotlight on a hotly-contested nomination.
The former federal prosecutor in Alabama has long been chased by past accusations of racial insensitivity, if not outright racism.
His nomination to a federal judgeship in 1986 was sunk over questions about insensitive comments and allegations that he disparaged civil rights groups.
Sessions is in line to lead the country's law enforcement amid searing debates over police interactions with minorities. The Justice Department is often called upon to investigate alleged abuses.
Given GOP control of the Senate, Sessions appears likely to be confirmed - but his nomination has nonetheless been one of the most charged of the new administration.
Protesters repeatedly interrupted the first hearing Tuesday, chanting "No Trump! No KKK! No fascist USA!"
Sessions rejected any accusations of racism, calling such claims "damnably false" charges. "I abhor the Klan and what it represents and its hateful ideology," he told the committee.
He later said: "I do not harbor the kind of animosity and race-based discrimination ideas that I was accused of. I did not."
Despite his hard-line views on immigration and law enforcement, Sessions is seen as a polite, amiable figure.
Sen. Lindsey Graham (R., S.C.), a member of the judiciary panel, doubted Booker's testimony would change any minds, because senators already know Sessions.
"I've known Jeff for 15 years, a lot longer than I've known Cory Booker. I don't think [Booker's testimony] will matter much to me," Graham said.
Others from both parties had mixed views on Booker's decision. Several said senators have the right to act how they see fit for their constituents, but stopped short of explicitly backing his move.
"It's not for me to approve or disapprove. He's asked to testify and his message is whatever he wants it to be," said Sen. Dick Durbin (D., Ill.), the Senate's second-ranking Democrat.
Another member of the Judiciary Committee, Sen. Jeff Flake (R., Ariz.) said he was "surprised."
"I'm not going to tell any senator what he needs to do, but I think if he sat down, worked with Senator Sessions, I think he'd probably have a different opinion," Flake said.
Sen. Tim Scott (R., S.C.), defended Booker's choice, while disagreeing with his views on Sessions.
"Each senator has a right to do what they think is in the best interest of the nation and of the citizens in their state," said Scott, the Senate's only African American Republican. "I certainly appreciate his willingness to be tenacious."