COMMERCE, Michigan (Reuters) - U.S.

COMMERCE, Michigan (Reuters) - U.S. Republican presidential candidate Mitt Romney took a dig at President Barack Obama on Friday over his birth certificate in comments that re-ignited a controversy over the Democrat's eligibility to be president.

"No one's ever asked to see my birth certificate," a smiling Romney told a rally of some 7,000 people in his home state of Michigan.

"They know that this is the place that we were born and raised," he said to the laughter of the crowd, speaking alongside his wife, Ann.

Romney's remarks drew a strong rebuke from Obama's team in the tight presidential contest that has become more ill-tempered in recent weeks. They also further obscured Romney's attempts to keep his message focused on jobs.

The joke was a reference to the widely discredited belief that Obama, whose father was from Kenya, was not born in the United States and thus is not eligible to be president.

Conservative fringes of the Republican Party -- including high-profile Romney backer Donald Trump -- have argued that Obama was not born in Hawaii as he says.

In an effort to end the "birther" controversy, Obama has released multiple copies of his birth certificate that show he was born in the United States. But some conservatives refuse to let the issue die.

"Governor Romney's decision to directly enlist himself in the birther movement should give pause to any rational voter across America," Obama campaign spokesman Ben LaBolt said.

Romney's comments came as Republicans geared up for Monday's opening of a four-day nominating convention, and were the latest detour from his central argument that Americans need a change from Obama's economic leadership.

'BIG BUSINESS IS FINE'

Less than 24 hours earlier, Romney said in Minnesota that "big business is doing fine in many places," seemingly contradicting his message that companies are struggling under Obama.

The line echoed an Obama comment about the private sector doing well that the Republican has repeatedly criticized on the campaign trail.

Romney has had a series of awkward campaign-trail moments -- highlighted by his European trip last month when he cast doubt on London's readiness for the Olympics and discussed cultural differences between Israelis and Palestinians -- that raised questions about Romney's political instincts.

"He seems to really struggle when he tries to be spontaneous," said Cal Jillson, a political scientist at Southern Methodist University. "He comes across as more awkward and tin-eared than malicious."

Romney attempted to turn the national conversation back to his record as a business executive in an op-ed published in the Wall Street Journal.

In a column titled "What I Learned at Bain Capital," Romney described his days at the private equity firm and said he would use his lessons from turning around private companies to fix the U.S. economy.

"My presidency would make it easier for entrepreneurs and small businesses to get the investment dollars they need to grow," Romney said.

Romney's camp sought to downplay his birth certificate remarks.

"The governor has always said, and has repeatedly said, he believes the president was born here in the United States," adviser Kevin Madden said. "He was only referencing that Michigan, where he is campaigning today, is the state where he himself was born and raised."

Romney brought up the issue despite having complained recently that the campaign had taken a nasty tone.

"Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them," Obama campaign spokesman LaBolt said.

(Additional reporting by John Whitesides; Editing by Alistair Bell and Vicki Allen)