Sen. Rand PaulRandal (Rand) Howard PaulDem wins Kentucky state House seat in district Trump won by 49 points GOP's tax reform bait-and-switch will widen inequality Pentagon budget euphoria could be short-lived MORE (R-Ky.) on Thursday took a swipe at billionaire businessman Donald TrumpDonald John TrumpAccuser says Trump should be afraid of the truth Woman behind pro-Trump Facebook page denies being influenced by Russians Shulkin says he has White House approval to root out 'subversion' at VA MORE, demanding to see his "Republican registration."

While speaking at a breakfast with New Hampshire Republicans one day after "The Donald" visited the Granite State, Paul riffed off the potential GOP presidential candidate's "birther" questions.

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“I’ve come to New Hampshire today because I’m very concerned,” said Paul, according to The New York Times. “I want to see the original long-form certificate of Donald Trump’s Republican registration.”

Paul's comments follow up on some GOP-aligned groups' effort to discredit Trump as a conservative. The free-market Club for Growth has accused Trump of being a liberal for his previous support of universal healthcare and his desire to raise tariffs on China. 

Trump, who is near the top of some polls, appeared in New Hampshire on Wednesday and spoke directly after President Obama released a copy of his long-form birth certificate to prove he was born in Hawaii, while calling people who question his citizenship "carnival barkers."

"Today I am very proud of myself, because I've accomplished something that nobody else was able to accomplish," Trump said

Paul, a Tea Party favorite, said it would serve the GOP better to get behind a candidate who has better conservative credentials.

“Let’s look to Republicans who not only talk the talk, but walk the walk,” he said. “If we find the right candidate, I see no reason why we can’t win in 2012.”

Paul, whose father, Rep. Ron Paul (R-Texas), is almost certain to run for president, has questioned Trump's potential candidacy before.

Asked by CNN in February about Trump's chances, the younger Paul said: "I think his chances are less than my father's."