Hillary Clinton will gain votes after Election Night. Here’s why.

By Charles Stewart III and Edward B. Foley for the Washington Post.

Most Americans assume that by the wee hours of tomorrow, the national media will declare (unofficially, but still decisively) either Hillary Clinton or Donald Trump our next president.

Of course, the 2000 election showed that the result might not be settled so quickly. Considering how tight the polls have been, one or two battleground states may be too close to call Wednesday morning.

That would be good news for Hillary Clinton’s chances. She’ll probably take more of the mail-in and provisional ballots that can’t be counted until the days and weeks after the election. Whatever her vote share tonight, it will probably increase in the weeks to come. Let us explain.

Good news and bad news about the chance the election will be decided tonight

For election administrators, 2000 was a wake-up call. Prompted by the controversial Bush v. Gore decision, states created uniform counting standards, which had been contested during the Florida recount. Reforms include safeguards that protect voters whose names were improperly removed (or never added) to registration lists, and procedures to ensure overseas citizens’ and service members’ votes will be handled equitably.

But some of those election administration changes make it much more likely that millions of votes won’t be counted until the days and weeks following Election Day.

Why? For two main reasons. First, more votes are now cast by mail. In 2000, approximately 10 percent of ballots were sent by mail. That had nearly doubled, growing to 19 percent, by 2012. In 2016, we expect that to be well over 20 percent. Some of these are tallied on election night, but not all. Many states allow absentee ballots to be counted if they are postmarked by Election Day.

Second, the Help America Vote Act passed in 2002 mandated provisional voting, which allows voters whose registration is questioned to still cast a ballot. That ballot is not counted immediately. It’s put in an envelope, much like an absentee ballot, and counted only if that voter’s registration is verified later. Resolving provisional ballots can take days or, as happened in this year’s California primary, weeks.

According to statistics from the federal Election Assistance Commission, in 2012, voters cast at least 2.7 million provisional ballots. The two states with the most provisional ballots, California and New York, aren’t battleground states. But the third and fourth-ranked states — Ohio and Arizona — are.

These “overtime” ballots tilt toward the Democrats

So more ballots will still need to be counted after Election Day. Our most recent research shows that these “overtime” ballots tilt in favor of Democratic candidates for president.

Consider that in the election returns reported by the New York Times on  Nov. 8, 2012, (which we use for the initial election night counts), nearly 118 million ballots had been counted. In those, Obama led by nearly 2.8 million votes.

But by the time all the states had finished their official canvasses several weeks later, the total ballot count included more than 126 million votes — an increase in 8 million votes since election night. And Obama’s lead had grown to well over 4.8 million votes.

In other words, 51.1 percent of the two-party votes counted on election night 2012 were for Obama. Of ballots counted after election night, 62.7 percent were for Obama. By the time all the votes had been counted, Obama’s share of the vote had grown to 51.9 percent.

We’ve examined such late-counted ballots going back to 1948. For most of the postwar era, the “overtime vote” changed little in either direction. One exception came during the 1960 election, when Richard Nixon won a 0.20 percentage point shift in his favor during post-Election-Day counting. The other exceptions reveal a notable trend: During 2004, 2008 and 2012, the Democratic candidates enjoyed gains of 0.12, 0.35, and 0.39 percentage points, respectively.

In other words, since 2000, votes counted after election night have increasingly favored the Democratic candidate. We call this the “blue shift.”

Here’s why the late count now favors the Democratic candidate. People who cast provisional ballots are usually those whose registration appears irregular on Election Day. Most registration questions come because voters have moved without updating their registrations. In recent years Democrats have won more of the votes from social groups that tend to move often: racial minorities and young adults.

Here’s what to expect from election night 2016

Tonight’s totals will almost certainly underestimate Clinton’s vote share, both nationwide and in most states.

If Clinton is ahead by early Wednesday morning, expect her margin of victory to widen as more votes are counted in the weeks to come.

If Clinton is behind early Wednesday morning, she might still catch up — especially if she is behind in battleground states likely to have large numbers of provisional ballots.

Which battleground states might hinge on late-counted ballots?

To help estimate which states these might be, we have compared the closeness of the current presidential race with the percentage of 2012 ballots that were provisional. Any state where the 2012 provisional ballot rate is close to the current polling margin may turn on late-counted ballots — that is, if Clinton is just barely behind late tonight.

Six states meet those criteria: Arizona, Colorado, Florida, Nevada, North Carolina and Ohio.

Watch these states as returns come in tonight. Make sure that the media does not call them prematurely without adequately considering the possibility that provisional ballots might affect the outcome. Remember that in 2000, most networks “called” Florida for Gore, which they then had to retract.

Of course, even if the election is close in one of these states, Clinton is by no means guaranteed victory from the provisional ballots. In 2004, George W. Bush led by 121,012 votes in Ohio, with 158,642 provisional ballots left to count. Only about three-quarters of these proved to be from eligible voters — and Bush took in a significant share of those, although a smaller share than did Kerry. Bush’s lead was too great to be surmounted by the provisional ballots.

Whatever the vote count tonight, Clinton’s share will probably grow in the next few weeks as states release their official tallies. That’s not because the system is “rigged.” It’s just the nation’s dedication to making sure that every vote is counted.