A Monmouth University poll released Tuesday, for example, shows Clinton ahead of Republican Donald Trump by 10 points in Arizona among early voters – but down a point among voters overall. And with early voting becoming more popular – nearly 9.5 million people had cast early ballots by mid-day Tuesday, McDonald estimates – there's no guarantee that even the early voter profile of previous campaigns will remain the same.

The most important comparison, McDonald says, is not the straight comparisons between Republicans and Democrats in raw, early returns, but in how well each party is doing compared to the same point in 2012. Clinton, he says, is doing well on that test along the Eastern Seaboard, including Maine, North Carolina and Florida, while Trump is overperforming in Ohio and Iowa – two states that have favored Democrats in recent presidential elections. (The early ballot tallies, importantly, only show the party registrations of the voters, and not how they actually voted.)