Senator Rand Paul (R-KY) mocked real estate mogul Donald Trump on Thursday, doubting his GOP credentials after Trump doubted President Obama's birthplace.

"I want to see the original long form certificate, with embossed seal, of Donald Trump's Republican registration," he told a crowd at the Merrimack County Republican Committee breakfast fundraiser in New Hampshire.

His father, Texas Rep. Ron Paul (R), announced the launch of his presidential exploratory committee on Tuesday.

Although Trump surged to the top of some Republican 2012 polls after raising doubts about Obama's birthplace and eligibility to be president, his former political leanings and policy positions on issues are likely to upset GOP voters.

"I am very proud of myself, because I have accomplished something that nobody else has been able to accomplish," Trump told reporters after Obama released his long-form birth certificate on Wednesday.