On Sunday, the Nov. 6 elections will be just 100 days away.

But that milestone has become something of a misnomer in an era when two states conduct elections strictly through the mail, voters increasingly vote absentee and many states allow early in-person voting that continues to grow in popularity.

As a country, we’ll collectively be glued to our television screens (or mobile devices) on the evening of Election Day to find out who won the White House and the battle for Congress. But to a significant degree, the outcome could be decided long before then, particularly in a few of the states that really matter.

Consider: Early voting in Ohio — perhaps the ultimate arbiter of the presidential race — begins 35 days before Election Day and runs through Nov. 2. That means Buckeye State voters can head to the polls on the first Tuesday in October (Oct. 2.). In Iowa, early voting begins even earlier — on Sept. 28. In Arizona, which could see a competitive Senate contest with implications for which party holds the majority in January, early voting begins Oct. 11.

“Historically, campaigns had around 14 hours to get voters to the polls on Election Day. Early and absentee voting expands that window significantly and fundamentally changes the rhythm of a campaign cycle,” said Phil Singer, a Democratic strategist and veteran of Senate and presidential campaigns. “Ads that get cut too early in a campaign don’t necessarily have much impact. But with early and absentee voting, an ad cut five months ahead of Election Day can have a real time impact that it wouldn’t otherwise have if voting were restricted to Election Day.”

This year, voters in some states will be able to send in their absentee ballots in as early as the first half of September, within a few days of the Sept. 6 conclusion of the Democratic National Convention in Charlotte, N.C. In North Carolina, absentee ballots are available beginning on Sept. 7. In Virginia and Wisconsin, absentee ballots are available beginning on Sept. 21 and Sept. 15, respectively.

Unlike years past, effective national and Congressional campaigns must now do much more than figure out who their voters are and what issues motivate them. Candidates and their advisers must gather data on when and how their supporters like to vote, and time their outreach and messaging to reach these voters just at the time they are preparing to cast their ballots.

“Early voting and absentee voting are sort of merging in the mind of operatives at this point as more and more early voting laws go into effect,” said a Republican campaign adviser.

Here’s a guide to when the elections really begin in a half dozen battlegrounds and other states that might be competitive. Some dates are approximate, but all have been verified with each state’s elections division or secretary of state’s office. Additionally, some states that do not have early voting allow for in-person absentee voting.

State 
             
Early Voting Begins
    
Absentee Ballots Available

Arizona              Oct. 11                                Oct. 11

Colorado           Oct. 22                                Oct. 15

Florida              Oct. 27                                Oct. 2

Iowa                  Sept. 28                              Sept. 28

Michigan           N/A                                    Sept. 22

New Hampshire    N/A                              Oct. 7

Nevada             Oct. 20                               Oct. 17

North Carolina    Oct. 18                           Sept. 7

Ohio                    Oct. 2                                Oct. 2

Pennsylvania     N/A                                 Oct. 23

Virginia              Sept. 21                           Sept. 21

Wisconsin        Oct. 22                              Sept. 20

Justin Worland contributed to this report.