Election Day is more than a month away, but voters are already casting ballots to pick their next president.

Voting opened on Monday in Wisconsin Rapids, Wisconsin, making the town’s voters among the first in the nation to do so. So far, voting is limited to a smattering of counties and municipalities in states that allow local governments to set their own schedules. On Friday, however, voters statewide in South Dakota and Minnesota will be allowed to cast early ballots. And other states are joining soon: New Jersey starts early voting on Saturday, and Vermont plans to start early voting as soon as the ballots are ready. (State rules say no later than Saturday.)

In total, 37 states and the District of Columbia will allow early voting in 2016. Some of those states allow voters to request mail ballots and send them in, while others open physical polling places weeks or even months before the election. In the 2012 presidential election, approximately 30 percent of the votes cast came via mail or early balloting, meaning many voters will weigh in before the candidates have had a chance to make their closing case.

Typically, Democrats lead in early voting, especially in-person early voting, because […]