Now we might be able to get a sense of how Democrats are performing in the early vote by looking at African-American turnout or overall Democratic turnout (in states with partisan registration), but we can’t know how independents are voting. We might make assumptions about this by looking at public polling, but then what value are we adding beyond what the public polling says? Plus we’re incorporating the error margins of public polling into our estimates, which will be even greater for demographic subsamples.
The real problem with this, however – and this is true with a lot of early voting analysis – is that for any of this to work we have to assume that the early vote is somehow representative of the Election Day vote in order to fill in the second half of the equation. The problem is, it isn’t. Research suggests that the early vote tends to be comprised of more partisan, higher propensity voters. In the most recent elections, they have often skewed Democratic, most likely as a side effect of increased Democratic emphasis on early voting (compare this with Donald Trump, who has been telling his supporters to vote on Election Day).
So basically, we’re left without really knowing how the early voting electorate is voting, without knowing how the Election Day electorate is likely to vote, and without knowing the size of the Election Day electorate. More importantly, we don’t know the effect to which campaign strategy is creating the appearance of a participation surge by merely cannibalizing Election Day voters by mobilizing voters who would have voted on Election Day anyway. This is a problem.