|Obama speaking to David Letterman [Photo: BBC]|
Earlier, Mr Romney defended his remarks after secretly filmed video of a speech to donors became public.
He told Fox News he knew those "dependent on government" would not vote for him in November's election.
Mr Romney also decried the notion of government "redistribution", calling it an "entirely foreign concept".
More leaked video emerged on Tuesday, showing Mr Romney saying Palestinians do not want peace in the Middle East.
The full video of the Florida fundraiser was also published by liberal investigative magazine Mother Jones.
In Tuesday's Fox interview, Mr Romney stood by his comments about the 47% of Americans who do not pay income tax. He said they support President Barack Obama and would never vote for him. He said his statement was "about the campaign".
"I'm talking about a perspective of individuals who are not likely to support me," he said. "Those that are dependent on government and those that think government's job is to redistribute, I'm not going to get them," Mr Romney said.
The real problem, he added, was that so many people were not eligible to pay income tax because they had fallen into poverty.
Recent polls indicate that the election is likely to be a close contest, although an NBC News/Wall Street Journal poll released on Tuesday evening showed that Mr Obama's approval rate has hit 50% for the first time since March.