Obama Leads Among Early Voters

Up to 40 percent of ballots may be cast before the election

Early voters could account for up to 40 percent of all voters in the 2012 U.S. presidential election, and polls of people who already have cast ballots show President Barack Obama with a comfortable lead over Republican challenger Mitt Romney.

Both candidates have been urging supporters not just to vote but to do it early as Republicans and Democrats campaign vigorously - particularly in key battleground states - to lock up as many votes as possible before Election Day on November 6.