U.S. President Barack Obama gained ground on Republican rival Mitt Romney for the third straight day, leading 46 percent to 43 percent, according to the Reuters/Ipsos daily tracking poll released on Tuesday.

Released hours before the presidential contenders face off for their second debate, the poll showed the number of undecided voters had increased, indicating a drop of support for Romney among the coveted voting bloc.

The poll showed that Obama has recovered some ground after a poor showing in his first presidential debate. After dropping below Romney in the wake of the October 3 debate, Obama regained the lead on Sunday and has increased that by 1 percentage point each day this week.

"I think what we're seeing is after the first debate Romney definitely saw a surge. A lot of his surge wasn't coming from Obama voters, it was coming from undecided voters, which is exactly where both candidates are seeking support," Ipsos pollster Julia Clark said.

"Now, we've actually seen the Romney bump decrease again a little bit," Clark said.

She said the number of people who have changed their minds and moved back to the undecided column took 2 percentage points off Romney's support and one point from Obama. Undecided voters now account for about 13 percent of likely voters, compared to 10 percent on Monday.

Romney and Obama meet at New York's Hofstra University on Tuesday night for a town hall-style debate where the questions will all be asked from undecided voters who have been selected to be part of the audience.

"It's a great place to be entering a debate tonight," said Clark, noting that the undecided voters will be able to ask questions on topics that interest them. "Tonight's debate will undoubtedly impact on some of them."

About 10 percent of the registered voters surveyed - a group of 178 people - said they had voted early. Among early voters, 59 percent said they had voted for Obama, while 39 percent said they had voted for Romney.

The online survey of 1,846 likely voters was conducted between Friday and Tuesday. The precision of the poll is measured using a credibility interval, which is plus or minus 2.6 percentage points for likely voters.