Voters casting early ballots in Iowa. Photo: Scott Olson/Getty Images

For all the excitement surrounding Election Day, it’s important to keep in mind that voting has already begun in many parts of the country, and will intensify before November 6. Early voting (by one of three methods: absentee ballots, in-person early voting, or non-absentee voting by mail) has roughly doubled as a percentage of total turnout since 2004 — from about 20 percent then to 40 percent in 2016. Early voting opportunities vary significantly by state, of course. Twelve states (including New York) only allow “excused required” absentee voting, which typically means a voter affidavit stipulating some accepted reason for being unable to vote on Election Day (generally travel or work). At the other end of the spectrum, three states (Colorado, Oregon, and Washington) conduct virtually all voting by mail.

In 16 states, over half of the vote was cast early two years ago; that includes such electoral powerhouses as California (59 percent), Florida (68 percent), Georgia (59 percent), and Texas (63 percent).

Illustration: U.S. Election Assistance Commission

The pace of early voting does vary significantly: Balloting began on October 8 in California (where all of the early voting is by mail ballot); today in Georgia (where in-person and absentee ballot early voting is roughly equal in magnitude); and not until October 27 in Florida (where in-person voting represents a majority of early voting). All told, early voting is now underway in 20 states. The total number of votes is small now, but will start ballooning soon:

10/15 #earlyvote update: At least 793,007 people have voted in the 2018 general election https://t.co/re0Ev4JdHh — Michael McDonald (@ElectProject) October 15, 2018

The strategic importance of early voting is obvious once you think about it: voter appeals (whether it’s ads, direct mail, door-to-door canvassing, or emails and social media organizing) must take into account the wildly varying “Election Days” that specific voters choose. Some campaigns focus on “banking” early votes with heavy expenditures aimed at getting favorable categories of voters to cast ballots as soon as possible. But it’s important to remember that this can give a misleading impression of how the overall election is trending, as this post-2016 note from Nate Silver points out:

In North Carolina, Clinton won the early vote by 2.5 percentage points, or about 78,000 votes. Furthermore, about two-thirds of votes were cast early. But Trump won the Election Day vote by almost 16 percentage points. That was enough to bring him a relatively healthy, 3.6-point margin of victory over Clinton overall.

In states where early voting is tracked by party registration and/or race, there is a tendency to follow reports as though they are early returns on Election Night. In some cases, they are just votes that would have otherwise been cast on Election Day, and tell you little about the outcome.

Another important factor to keep in mind is that early voting doesn’t just move Election Day up for those who use this option: It also (at least in terms of voting by mail) stretches Election Night out for days and even weeks. And at least two states with heavy voting by mail (California and Washington) count votes postmarked on Election Day that come in later. They also tend to count later mail ballots after they count everything else. In California’s June 5 primary, about 30 percent of the vote (more than 2 million ballots) were not counted by the end of Election Night.

If the battle for control of the House is very close, the seven competitive House races in California and the three in Washington could make any early resolution difficult — and not just because these are Pacific Time Zone states. Traditional notions of Election Day and Election Night are increasingly misleading.