In California around 70-75% of all the votes are counted on election night. The remaining votes are VBM ballots returned on election day, provisional ballots, and damaged ballots that need to be examined to see if they are valid. In 2012, Democrats in California improved their results in the post election day counting. It was across the board and the gains were often substantial. Republicans lost several districts in post-election day counting. It wasn’t the same in the 2014 primary, however, and Republicans gained in most post-election day counting.
I tracked the post-election day count in 31 races that people thought might be competitive. Those included 14 congressional, 13 assembly, and 4 state senate races. Democrats did better in 30 of them in the post-election day ballot count. The lone exception was CA-3. There is still one California county which hasn’t finished counting and that county is partially in CA-3. It’s a Democratic leaning county which is divided between CA-3 and CA-5. So it’s possible that Democrats might do better in every competitive contest.
How much Democrats did better varied widely. In AD-57, the Democrat got 51.8% of the post-election vote after getting 51.4% on election day. At the other end was AD-32. The Democrat had 53.1% on election night, but got 64.9% in post-election day ballots. Democrats had the biggest post-election day improvement in CA-7, 16, 21, 26, 41, and 52. Coincidentally, they trailed in 4 of those districts and led by a small amount in a fifth. As with 2012, Democrats post-election day never changed the lead more than 4.0%. So the rule of thumb that a Republican who has 52.0% or better on election night wins held. There were 3 congressional districts and 2 assembly districts where the Republican led, but had less than 52.0% on election night. The GOP lost all 3 congressional districts, but won both of the assembly districts.
People have theorized that Democrats do better post-election day because they are far more likely to turn in their VBM ballot on election day than Republicans. I’m not entirely sure why this would be the case, but it’s as good a theory as any. What we should keep in mind for 2016 is that any Republican under 52.0% on election night is in danger of losing.