And in three states -- Idaho, South Dakota and Vermont -- voters already can show up in person to vote.
Wyoming begins its in-person voting Thursday, as does the presidential battleground state of Iowa. In some of the other hotly contested states -- Colorado, Nevada, North Carolina and Florida -- more than half the ballots are expected to be cast early.
President Barack Obama pulled off an aggressive early mobilizing strategy in 2008 that overpowered Republican challenger John McCain.
This year looks different. GOP nominee Mitt Romney is pouring manpower and money into a push to sew up early votes.