Republican challenger Mitt Romney held a 1 percentage point lead over President Barack Obama in Thursday's Reuters/Ipsos daily tracking poll in a presidential race that is effectively a dead heat less than two weeks before Americans vote.

In a repeat of Wednesday's results, Romney led Obama among likely voters by 47 percent to 46 percent, a statistically insignificant margin, in the four-day online tracking poll.

The race has tightened since early October, around the time of the first of three televised debates between the two candidates. With Romney and Obama barnstorming the key battleground states of Ohio, Florida, and Virginia on Thursday, 13 percent of registered voters and 30 percent of independents said their vote remains up for grabs in the November 6 election.

Both candidates are putting an emphasis on early voting. Obama will fly to Chicago on Thursday to cast his early vote. Among registered voters, 18 percent said that they have already voted, with 55 percent voting for Obama and 40 percent for Romney. Another 31 percent said they planned to vote before Election Day.

The accuracy of Reuters/Ipsos online polls is measured using a credibility interval. The credibility interval on the survey of 1,168 likely voters is 3.3 percentage points.