Most of them voted during a period when Clinton had a national lead — a narrow lead, if they voted a few weeks ago or a large lead if they voted more recently. So while it's probably not a surprise that early vote tallies in several swing states show a shift to the Democrats since 2012, it still means that Clinton has a greater percentage of banked votes than President Obama did at this point four years ago.

Catalist, a voter data firm that works mostly with Democratic campaigns, provided The Washington Post with early vote numbers from several battleground states that allowed us to compare current returns with the number of ballots returned in years past. In seven states for which returned ballot data was available by party, Democratic ballots made up a larger percentage of what had come back by the 20-day mark (that is, by 20 days before Election Day) than in 2012 (or in 2008 for Florida). In some cases, like Arizona and North Carolina, the shift to the Democrats was substantial.

(The data from Colorado so far includes only overseas and military ballots.)

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Take North Carolina, for example.

By this point four years ago, more than half of the returned ballots came from Republican voters. (Note the percentage graphs at the bottom.) This year, the split is much more even. Republicans are still a plurality of the returns, but the margin is narrower.

The Clinton campaign has put a heavy focus on targeting the early vote in North Carolina and elsewhere, which may help play a role. But notice, too, that far fewer Republicans have returned ballots so far — compare the red line under 2016 with that under 2012 (or 2008).

In Iowa, where the partisan split is essentially the same as 2012, both parties have seen lower returns so far this year. That's interesting in part because polling suggests that Iowa is much more supportive of the Republican this year than four years ago. As it stands, Trump leads by 3.3 points, according to the RealClearPolitics average (which is a bit out of date). Four years ago, the margin was similar, in favor of Obama.

Nevada has seen fewer returns come in this year than four years ago. By that point in 2012, there'd been a big spike in ballots returned by Democrats, and that hasn't happened this year.

In Florida (for which 2012 data wasn't available), the margin between the parties is much narrower, while the number of returned ballots is much larger.

In Ohio, far fewer ballots have been returned than in 2012. It's a reminder that a lot of factors play a role in the differences between election cycles. Early voting is a frequent point of debate between the parties and, by extension, in the courts. Ohio's Republican legislature rolled back a week of early voting during which voters could both register and vote on the same day, for example — a change that's still being fought in court.