California Senate President Pro Tem Darrell Steinberg floated the idea of having the governor fill legislative vacancies awhile ago, but only now is he going forward with legislation. Previously he failed to mention that the governor would have to fill the vacancy with a member of the same party, leading people to believe he was trying to sneak in seats for Democrats. That is apparently in the legislation now.
Even so, it'd still be a negative for the GOP. Republicans turn out at a much higher rate in specials here than Democrats do. If a Republican has half a chance, he'll likely win a special. This cycle a Republican won a 62% Obama seat, came within 1% of winning a 65% Obama seat, and within 3% of a 66% Obama seat. So while Steinberg says this bill is for the state to save money with low-turnout elections, it staves off any potential Democratic losses between elections. Of course this'd only apply to legislative elections. A congressional vacancy would still be filled with a special election.