WASHINGTON, Oct 14 (Reuters) - U.S. 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.

Early voting was a big part of Obama's victory over Republican John McCain in 2008, and his campaign aims to repeat its success this year.

The Reuters/Ipsos poll indicates the campaign's efforts appear to be paying off, although its advantage could erode as Election Day approaches.

The Obama campaign says it is leading among early voters in Iowa and Ohio, and trailing by a smaller margin than 2008 in several other swing states. It expects its early voting efforts will help the campaign weather a blitz of negative ads expected to saturate the airwaves in battleground states in the final weeks before Nov. 6.

In Iowa, about twice as many registered Democrats as Republicans have voted by now - a potential warning sign for the Romney campaign, he said.