In the interview with NBC News, Paul took particular aim at Trump’s recently released immigration proposal, which he said made “no sense” and is “bizarre.”

“Even the people who think they might like him, when they hear what his bone-headed plan is, you think they might have second thoughts,” he said.

Still, one element of Trump’s immigration plan that Paul won’t push back on is his proposal to end birthright citizenship.

Back in 2011, Paul co-sponsored a constitutional amendment to put an end to the right that allows everybody born on U.S. soil to be a citizen. He told NBC News that today’s circumstances still warrant that move.

“What I would say is, if you have an open border you can’t have birthright citizenship,” he said.