Washington (CNN) We're still a month out from the first Tuesday in November, but voters have already begun heading to the polls in several key states for early voting.

Which means that campaigns are already shifting to get-out-the-vote mode -- this week alone, early voting kicks off in these states:

Early voting, which is different from absentee voting, allows voters to cast their ballots in person during any period before Election Day, for any reason.

Driving intrigue -- and turnout -- are a number of high-profile Senate, House and gubernatorial races in early voting states. Case in point: CNN's key races like the House races in OH-01 (where GOP Rep. Steve Chabot faces a challenge from Democrat Aftab Pureval), CA-10, CA-25 (home to incumbent GOP Rep. Steve Knight and Democratic challenger Katie Hill), CA-39, plus the Senate races in Arizona and Indiana (where incumbent Democrat Joe Donnelly is trying to hold onto his seat ).

And when it comes to in-person early voting, Democrats tend to perform better, as opposed to Republicans, who tend to run up their numbers in turnout from mail-in ballots and same-day voting. Early voting expert Michael McDonald , an associate professor of political science at the University of Florida, tells me there's a pretty reliable partisan breakdown among who turns out for early voting., as opposed to Republicans, who tend to run up their numbers in turnout from mail-in ballots and same-day voting.

Not every state has the same early voting standards, so these aren't ironclad rules. But McDonald said his rule of thumb is: If it looks that Democrats are losing the early vote, it generally means they've lost the election.

Instead of a daylong sprint to get as many people to the polls, early voting means the get-out-the-vote process is now a marathon, lasting weeks rather than hours. It also allows campaigns to be more flexible with their campaign funds, identifying potential voters, turning them out, and then allocating funds to identify even more would-be voters.

Another state to watch: Nevada. As CNN's . As CNN's Eric Bradner writes, "My favorite early voting state, which doesn't come online until later this month, is Nevada, because everybody votes early there too and the data is so good that we'll know with near certainty the outcome of the races for governor, Senate and two House seats by the Monday afternoon before Election Day."