No Senator had ever testified against a fellow Senator during a cabinet confirmation hearing. Until yesterday.

Day One of Alabama Senator Jeff Sessions’s hearings to become Attorney General have unfolded under a tense cloud of racial unease, amplified on their second day by New Jersey Senator Cory Booker’s unprecedented decision to step before the Senate Judiciary Committee and call Sessions out.

Booker voiced concerns about what he called Sessions’s history of “open hostility” to civil rights matters, with nearly a dozen members of the Congressional Black Caucus seated behind him in support. Booker said he was doing it for the people. Critics of the move, who view Sessions’s confirmation as all but guaranteed, felt Booker was doing it to bolster a presidential run in 2020.

“He will be expected to defend voting rights, but his record indicates that he won’t,” Booker said of Sessions. “He will be expected to defend the rights of immigrants and affirm their human dignity, but his record indicates that he won’t.”

Booker built his testimony on the idea that the Attorney General is a crucial component to the nation’s need to deconstruct systemic bias in its criminal justice system. He argued, “[Sessions’s] record indicates that we cannot count on him to support state and national efforts toward bringing justice to a justice system that people on both sides of the aisle readily admit is biased against the poor, drug addicted, mentally ill, and people of color.”

Republican criticism of Booker’s (grand)stand was rampant, most directly expressed by rising GOP star Tom Cotton, the Arkansas Senator who posted to Facebook, “I’m very disappointed that Senator Booker has chosen to start his 2020 presidential campaign by testifying against Senator Sessions.” He went on, “This disgraceful breach of custom is especially surprising since Senator Booker just last year said he was ‘honored to have partnered with Senator Sessions’ on a resolution honoring civil-rights marchers.”

Sessions argued that if a Senator disagrees with a nomination, they are free to vote against it. “But what is so unique about those views to require [Booker’s] extraordinary testimony?” Cotton asked. “Nothing. This hearing simply offers a platform for his presidential aspirations. Senator Booker is better than that, and he knows better.”

No doubt, Booker’s extraordinary break with tradition is undoubtedly an attention-generating move for someone highly tipped to run for president in 2020. In interviews later in the day, the New Jersey Senator did his part to deny political aspirations being a factor, focusing fully on criminal justice reform and equal protection under law, calling his decision to testify a matter of “conscience and country.” Booker’s testimony was followed by that of civil rights icon and Georgia Representative John Lewis who urged the Judiciary Committee to set aside collegiality.

“It doesn’t matter how Senator Sessions may smile, how friendly he may be, how he may speak to you,” said Mr. Lewis. “We need someone as attorney general who’s going to look out for all of us, and not just some of us.”

Republicans countered with images of Lewis and Sessions walking together over the Edmund Pettus bridge in Selma on the 50th anniversary of its crossing by civil rights activists, and with testimony of African-Americans who have worked professionally with Sessions and doubt claims that the Alabama Senator is a racist.

In the final analysis, Sessions is almost certainly a lock for Attorney General. Will Booker run for President in 2020? It’s getting closer by the day, with each moment in the spotlight, and with each Facebook post.