Obama, after meeting with emergency officials at the White House to discuss the response to Hurricane Sandy, flew to Iowa and then on to Wisconsin. His schedule also included Ohio and Virginia.
Romney began the day in New Hampshire and then also went to Wisconsin. Both candidates visited Dubuque, Iowa.
The election is clearly not going to be a repeat of 2008, when Obama defeated U.S. Sen. John McCain, R-Ariz., by more than 7 percentage points and by 365 votes to 173 in the electoral college, The New York Times said. It could be more like 2000, when George W. Bush became president after losing the popular vote to Al Gore and eking out a small margin in the electoral college when the U.S. Supreme Court ordered a recount of the Florida vote be halted.
Polls show Obama and Romney very close in many states, including Wisconsin, which has not voted for a Republican presidential nominee since 1984. Romney's running mate, Paul Ryan, is a Wisconsin congressman.
"It's always tantalizingly close for Republicans, and I assume that's where we are at with this one," said James E. Doyle, a Democratic former governor.
In Iowa, the Republican governor, Terry Branstad, forecast a Romney victory, saying Obama was attempting to "rekindle" an enthusiasm for him that has evaporated.
"Initially, there were a lot of people who were against Obama and weren't that wild about Romney, but over the last few months that has changed, and now there is really genuine enthusiasm for Romney," Branstad said.