CHICAGO - As Republican presidential nominee Mitt Romney works to regain momentum in the final weeks before the Nov. 6 election, time is pressing. Election Day essentially starts this week in Iowa.

Voters in Ohio similarly can begin casting ballots on Oct. 2, in North Carolina on Oct. 18 and Nevada on Oct. 20. In total, six of the nine top battleground states will have early, in- person voting underway by the third debate between Romney and President Obama on Oct. 22.

That means the candidates could deliver the best debate performances of their careers and it would be too late to sway the votes of a growing slice of the electorate.

Although it brings additional logistical hurdles, early balloting benefits campaigns organized enough to take advantage of it because resources can be more finely targeted at the remaining voters the weekend before the election.

"This allows them to bank votes," said Michael McDonald, a professor at George Mason University in Fairfax, Va., who studies early voting. "They encourage their supporters to vote early and scratch out the names of people who have voted. Once they turn that ballot in, they're not going to contact them anymore."

All of the key battleground states except New Hampshire and Virginia allow early, in-person voting, while all provide absentee ballots before Election Day. In total, 32 states and the District of Columbia offer some form of early, in-person balloting, according to the National Conference of State Legislatures.

The voting in Ohio starts the day before the first presidential debate in Denver. That is one reason Romney and his running mate, Rep. Paul Ryan of Wisconsin, are hitting the state this week for a three-day bus tour.

In the past decade, states have dramatically expanded forms of early voting, pushed by residents living in an era of frequent travel, busy family schedules and the expectation of less standing in line because of technology improvements.

That expansion has also come with many legal challenges over the duration and mechanics of early voting.