This would be the first time in Senate history that a sitting senator will testify against another sitting senator for a Cabinet post during a confirmation.
"I do not take lightly the decision to testify against a Senate colleague," Booker said. "But the immense powers of the attorney general combined with the deeply troubling views of this nominee is a call to conscience."
Sessions' confirmation hearings, which started Tuesday
, are expected to raise additional questions on old allegations of racism from his past. When Sessions was a 39-year-old US attorney in Alabama, he was denied a federal judgeship because the Senate Judiciary Committee heard testimony during hearings in March and May 1986 that Sessions had made racist remarks and called the NAACP and ACLU "un-American."
Booker told CNN on Tuesday morning shortly before Sessions' hearing started that it was "consequential moment."
"This is one of the more consequential appointments in American history right now given the state of a lot of our challenges we have with our policing, a lot of challenges we have with race relations, gay and lesbian relations," Booker said.
Booker called Sessions' record "concerning in a number of ways," citing his opposition to bipartisan criminal justice reform and immigration reform, criticism of the Voting Rights Act and his "failure to defend the civil rights of women, minorities, and LGBT Americans."
Donald Trump supporter Rep. Chris Collins defended Sessions Tuesday and said "Cory Booker is all about the latest stunt."
"If you remember what he did when he was mayor in New Jersey, the first thing he does is he tries to grab international headlines," Collins told CNN's "New Day." "What he's doing today every -- never before in the Senate, it doesn't surprise at all."
He added, "What Cory Booker is doing is nothing but self-serving grabbing headlines. Jeff Sessions is going to answer those questions."
Booker has been known to work across the aisles to pass Senate legislation and worked in the Senate to restore provisions of the Voting Rights Act that were struck down by the Supreme Court.
Democratic Rep. John Lewis, a civil rights icon, is also expected to testify against Sessions during his confirmation hearings. Lewis and Sessions were pictured
holding hands during a march in Selma, Alabama, in 2015.
Sessions has allies from his own party. Sen. Tim Scott, the Senate's sole African-American Republican member, released a statement of support for Sessions Monday night in advance of the hearing.
"We may not agree on everything, but you would be hard pressed to find a nominee for any post that any Senator is in 100 percent agreement with," Scott said in a statement. "I have gotten to know Jeff over my four years in the Senate, and have found him to be a consistently fair person. I will continue working for what I believe is in the best interest of my state and my nation, such as criminal justice reform and stopping illegal immigration."