Andrew Sullivan highlights the significance of early voting in U.S. Presidential Elections (link). His second quote (from The Fix) requires a visit to your statistical adviser.
Andrew considers this an "important point":
Democrats generally vote early more than Republicans. In the five 2012 swing states where a 2008 early voting breakdown is available, Democrats voted early more than Republicans in all five. Even in a very good GOP year in 2010, Democrats voted early more often than Republicans in North Carolina, Iowa and Nevada.
I'm afraid many will misinterpret this statement. What really matters is which party succeed in pushing more wavering voters to vote for them early, and thus nail down those votes. It would give Democrats no advantage if those early voters are hardcore party-line voters. The hardcore Democrat can cast his/her vote on Election Day or any day prior but it's still one sure vote.
In other words, not every early voter is alike. Early voters who are independents, previously undecided, wavering, etc. are more useful than early voters who are party-line voters.
There may be an adverse selection effect, in the sense that undecided voters are less likely to vote early since, well, they haven't made up their minds. So the people who are voting early are really the ones you have in your bag.
For me, the significance of early voting are these two points :
- Early votes skew later voting propensity (which is why I don't think it's a good idea). This is similar to the East Coast/West Coast issue: by the time people in the West Coast wake up, there would already be results from the East Coast, or at the minimum, results from exit polls. This can have the effect of causing some people to stay home or other people to rush out to vote. It also allows campaign managers to take remedial action before Election Day.
- Early votes is an insurance policy. It guards against gaffes leading up to Election Day.