Story highlights Booker is expected to testify Wednesday

Sessions' confirmation hearings begin Tuesday

Washington (CNN) Democratic Sen. Cory Booker is set to testify against Republican Sen. Jeff Sessions Wednesday in an unprecedented move during his attorney general confirmation.

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."

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