Republican presidential candidate Sen. Rand Paul went after rival Sen. Ted Cruz (R-TX) by questioning his conservative authenticity.

“Ted’s just not genuine on some of these issues, he’s not authentic,” Paul said in an exclusive interview with Breitbart News.

Paul hit Cruz for failing to show up in the Senate to vote to last week for Paul’s “Audit the Fed” legislation.

The bill needed at least 60 senators to move forward with debate, but failed after receiving only 53 votes, when Democrats effectively blocked it. “I think he kind of deserted us when we needed him on the issue,” Paul said. “I think the liberty movement senses that he deserted us on the issue.”

At the time of the vote, Cruz was campaigning in New Hampshire. “It was very important,” Paul said. “This isn’t just a run of the mill vote. I worked for five years to get this vote.”

Paul also hit Cruz on for not being “genuine” when it comes to his position on the collection of mass telephone surveillance by the National Security Administration’s (NSA). “With the NSA vote, he gets on the stage as soon as he’s attacked by Rubio, he responds by saying ‘well, I voted for it so the government can collect 100 percent of your cell phone records. He wants to have it both ways.”

Paul told Breitbart News that Cruz sometimes purports to be a libertarian on the issue, but changes position when confronted by Sen. Marco Rubio.

Paul also hit Cruz on his comments about bombing ISIS to see “if sand can glow in the dark.”