Speaking Thursday morning at Merrimack County Republican Committee breakfast funder at Concord Holiday Inn, the Kentucky senator said: "I want to see the original long form certificate, with embossed seal, of Donald Trump's Republican registration."
After laughter subsided, he went on.
"I'm going to believe it when I see the embossed seal on his Republican registration," he said.
Paul is in the first in the nation primary state to quietly back his father, Texas Rep. Ron Paul, who is exploring another run for the White House. And, perhaps, lay the groundwork for a future run himself.
Trump was also in the state Wednesday.
Speaking with reporters after his remarks, Paul said he wanted to counter the perception that Trump's rise in the polls was a consequence of "tea party" activists' support for his "birther" remarks.
Michael A. Memoli contributed from Washington.