Mitt Romney's Sons Make Birther Joke, Claim GOP Hopeful Doesn't Know If He'll Release Tax Returns (VIDEO)

During a New Hampshire campaign stop for their father on Thursday, one of Mitt Romney's sons joked that the Republican presidential candidate would release his tax returns if President Obama released his grades and his birth certificate.

"I heard someone suggest the other day that as soon as President Obama releases his grades and his birth certificate... then maybe he'd do it," Matt Romney said in an exchange that was captured in exclusive video by Patch.com.

"He has not said that he will not do it, and he's also not said that he will," Matt Romney said. "I don't know the answer to that. I'm not sure he knows the answer to that, but he'll do everything that he needs to. He's certainly not afraid of anything, he's not hiding anything."

The Republican presidential hopeful has refused to release his tax returns, prompting the Democratic National Committee to question what Romney is "hiding" in a new video.

After the younger Romney made the birther joke, Tagg, another of Romney's sons, quickly chimed in to note "that was not my dad saying that."

"No, no no, that was just a suggestion," Matt added.

Obama did release his birth certificate in April of this year, but has still received scrutiny from birthers who believe the document isn't authentic. One critic, "birther queen" Orly Taitz, attempted to challenge Obama's New Hampshire ballot eligibility over the issue, but was rejected by the state's Ballot Law Commission.

Click here to read more on Romney's sons' New Hampshire appearance from Patch.

UPDATE: 1:20 p.m. -- Mitt Romney's son Matt Romney sent out his first tweet on Friday from the account @Matt_Romney to apologize for his comment about President Barack Obama's birth certificate earlier in the day.

"I repeated a dumb joke. My bad. RT @emilyrs Romney's son Matt made an Obama birth certificate joke on the trail in NH," he wrote, retweeting a post from Politico's Emily Schultheis.