One of Mitt Romney's sons did his best on Thursday to promote the rumor that the nation's first black president may not be a U.S. citizen.

Stumping for his dad in Concord, New Hampshire, Matt Romney told a group of senior citizens that the multimillionaire candidate might not release his tax returns until President Barack Obama releases his birth certificate, a document that was made public long ago.

"He has not said that he will not do it," the younger Romney said of the tax returns, sounding like the son of a politician. "He has also not said that he will. I think it's just a matter of time until that issue comes up. So, I don't know the answer to that. I'm not sure he knows the answer to that."

While it wasn't a particularly insightful answer, things probably would have been fine if Matt Romney had just stopped there -- but he didn't.

"I heard someone suggest the other day that as soon as President Obama releases his grades and birth certificate and sort of a long list of things then maybe he'd do it," he added.

Sensing that Matt had gone to far, the slightly-older Tagg Romney, one of the candidate's other sons, volunteered that his dad had not personally made that remark.

"No, no," Matt Romney agreed. "That's just a guess from someone else."

The president's Twitter account wasted no time in responding: "Mitt Romney's son thinks President Obama should release his birth certificate. Guess he doesn't have one of our mugs?"

Earlier this year Public Policy Polling found that the former Massachusetts governor was in fourth place among so-called birther Republican voters who believed that Obama was born in another country.

In April, the candidate declared that "the citizenship test has been passed."

"I believe the president was born in the United States," Mitt Romney told CNBC's Larry Kudlow. "The man needs to be taken out of office but his citizenship isn't the reason why."

The tax return issue may have become a sore point for the Romney family after the Democratic National Committee (DNC) suggested the candidate had something to hide.

"It would show that on the millions of dollars in income he enjoys each year, Mitt Romney pays a lower tax rate than teachers, fire fighters, police officers or other middle class wage earners," DNC National Press Secretary Melanie Roussell predicted in a media advisory.

"Mitt Romney will tell you that it's not required by the law that he release his returns but when he's advocating for policies that benefit the wealthy and the well-heeled, voters have a right to know what conflicts he might have with his own finances."

Watch this video from Concord Patch, uploaded Dec. 30, 2011.

Photo: Flickr/Talk Radio News Service

(H/T: Concord Patch)