(CNN) Sen. Cory Booker on Monday called out Sen. Bernie Sanders over his support for restoring voting rights to convicted felons still serving prison time, drawing a rare explicit contrast with one of his 2020 rivals.

In an interview with PBS Newshour , Booker called the debate over voting rights for incarcerated felons "frustrating" and suggested it distracts from the larger, more urgent question of mass incarceration, which has been a policy focus of Booker's campaign.

"So, if Bernie Sanders wants to get involved in a conversation about whether Dylann Roof and the (Boston) Marathon bomber should have the right to vote, my focus is liberating black and brown people and low-income people from prison," Booker said, recalling mass attacks in recent years. Roof killed nine black churchgoers in South Carolina in 2015; the Boston Marathon bombing killed three people and injured hundreds others in 2013.

The New Jersey Democrat added: "My focus is tearing down the system of mass incarceration, so that we don't even have to have the debate about people's voting rights, because they're not going to prison in the first place. People that don't belong there are there, and I'm going to stop that as president."

A Sanders campaign aide told CNN later Tuesday: "We're proud to stand in solidarity with major civil rights groups in unequivocally declaring that the right to vote is inalienable." The comment was in reference to a joint letter from civil rights groups encouraging candidates to support extending voting rights to people in prison.

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