Ned Foley and Charles Stewart WaPo oped:

Moreover, it would be reasonable for a candidate behind by several thousand votes in a state critical to winning the electoral college to think it possible to make up the difference by provisional ballots. Our research shows that in 2012, President Obama extended his margin of victory in multiple battleground states by more than 20,000 votes during the time between election night returns and final certification of the results. Obviously, these gains didn’t make a difference in the outcome, because Obama already had won. But they show that votes counted after Election Day could determine the outcome of a much closer election.

 Gains of this magnitude are much larger than what routinely occurred before the enactment of Help America Vote. While other factors may play a role, such as increased reliance on mailed ballots, our statistical analysis indicates this increase is largely due to provisional voting. It also explains why the overtime vote tends to favor Democratic candidates, whose voters are disproportionately affected by the kind of circumstances (such as a change of address) that can cause the need to vote provisionally. In other words, an election that is not decided on election night is more likely than not to end up favoring Hillary Clinton.

Because no count is officially complete until it includes all valid provisional ballots, every state has a well-rehearsed operating manual for conducting this procedure. Missouri’s experience in 2008 is instructive. It took 15 days to determine that John McCain narrowly beat Obama there (although the outcome didn’t alter Obama’s overall electoral college victory). Reviewing Missouri’s provisional ballots, which kept the state “too close to call” for two weeks, proceeded without difficulty, and there is good reason to think that this model could work even when an overtime state was necessary for reaching an electoral college majority.

Thus, if this year’s presidential election ends up being closer than previously expected, we should be prepared for the possibility that provisional ballots take us into extra innings. This won’t be a sign that the system is “rigged,” but that it’s working as designed.