Without outright drawing a line to Vice President Joe Biden, U.S. Senator Cory Booker (D-NJ) tonight threw a delicate elbow at the front-runner when he invoked the late Senator Strom Thurmond of South Carolina.

In an attack oblique enough to make fellow contender Julian Castro’s offensive look like shock and awe by comparison, nice guy with an ulterior motive Booker noted Thurmond’s longest filibuster in the history of the nation against civil rights.

Mired in single digits, Booker mentioned Thurmond at the ABC News-Univision Democratic Presidential Debate in connection with his argument that, as a denizen of urban America, he would advocate for racial justice unlike any of the other people onstage.

“We have systemic racism that is eroding our nation, our healthcare to the criminal justice system,” Booker pointed out. “We have a criminal justice system that is so racially biased. We have more African American under criminal supervision today than all the slaves in 1850.”

The Thurmond dredge contrasted with fellow rival Senator Kamala Harris’s frontal assault on Biden in another debate this summer, which landed the California senator post debate headlines but did little to give her a sustained bounce.

A failed Dixiecrat candidate for president in 1948, Thurmond mounted a 1957 filibuster on the floor of the senate that lasted 24 hours and 18 minutes.

Biden, who earlier this year incurred the ire from his Democratic presidential rivals when he mentioned working across the aisle with segregationists, in fact, delivered one of the eulogies at the late Senator Thurmond’s 2003 funeral.

Noting his late colleagues accomplishments at the time, Biden pointed to “Thurmond’s later years and his support for extending the Voting Rights Act and establishing a Martin Luther King Jr. holiday. Biden also noted, in an admiring context, a 1947 editorial that praised Thurmond’s work with reading programs for black students at the ‘separate, but equal schools’ that were a defining and, ultimately, unconstitutional function of racial segregation, according to BuzzFeed.

Booker’s mild attack contrasted most starkly tonight with former HUD Secretary Castro’s “ruthless” attempt at taking down Biden.