WASHINGTON (Reuters) - President Barack Obama and Republican challenger Mitt Romney are neck and neck in opinion polls, but there is one area in which the incumbent appears to have a big advantage: those who have already cast their ballots.

Obama leads Romney by 59 percent to 31 percent among early voters, according to Reuters/Ipsos polling data compiled in recent weeks.

The sample size of early voters is relatively small, but the Democrat's margin is still well above the poll's credibility interval - a measurement of polls' accuracy - of 10 percentage points. (full graphic: http://bit.ly/RmeEen)