Paul said mass incarceration is the "new Jim Crow." EPA/SHAWN THEW

Sen. Rand Paul (R-Ky.) said that Bill and Hillary Clinton were "proud" to be in power at a time when legions of young black men were put in prison as part of the war on drugs.

"Bill Clinton presided over the incarceration of an entire generation of young black men," Paul, a GOP presidential candidate, said Thursday on "The Wilkow Majority" on SiriusXM. Young black men, Paul said, are being put in jail "at a rate never before seen in history" because of the war on drugs.

"Hillary Clinton, Bill Clinton, they were all proud to do this, but now that I've been speaking out and saying that mass incarceration is the new Jim Crow, now all the sudden the Clintons are saying now we’re going to be back on the other side of this issue now," he said.

Paul has long emphasized criminal justice issues, for example co-sponsoring legislation that would reform mandatory sentencing laws.

[Rand Paul’s criminal justice challenge — and how he might deal with it]

He pointed to poll numbers that showed him doing well against - or surpassing - Hillary Clinton in some swing states. A recent Quinnipiac poll, for example, showed Paul running even with Clinton in Ohio and ahead of her in Pennsylvania

"Someone from the DNC is listening to our radio interview now and they’re looking for ways to attack me because they see me as a threat to Hillary Clinton," Paul said. "I’m going to the south side of Chicago, I’m going to the inner city Philadelphia, I’m going to Baltimore, I’m going to Ferguson and I’m saying what have the Democrats done for you? What have they done to alleviate poverty? What have they done on crime? What have they done for the young men in your community and you know what it’s starting to gain traction."

Paul went to Chicago last month, spoke to the Baltimore County GOP earlier this month, and spoke at the Constitution Center and held a rally outside Independence Hall in Philadelphia last month.