The L.A. Times reports that Republicans fear that a weak Republican gubernatorial candidate could lead to down ticket losses. Is that true?
The presence of coattails is regarded as a given. Certainly there have been years where a party's Presidential candidate does extremely well and that party does well down ticket. On the other hand, Republicans lost a number of senate races in 2012 in states where the voters punched Mitt Romney's name, sometimes by huge margins. I've seen very little about whether a gubernatorial or senate candidate has similar coattails, although these people seem to think they do.
There are two different coattails voters. The first is a swing voter who decides to vote with a particular political party down ticket based on how they feel about the top of the ticket. I know this is conventional wisdom, but I really question it. Swing voters aren't sheep who just punch a button down ticket for who they like at the top of the ticket. They may decide that they dislike a party (Republicans in 2008 and Democrats in 2010) and want the other party in office, but that may influence their top of the ticket vote more than the other way around. I don't dismiss there are some voters like this, but not as many as people think.
The second coattail voter is the casual voter. Every election gets some voters who vote in some elections, while others take an election off. These voters can be dedicated to one party and vote for that party. In 2008, some casual Democratic ticket voters showed up, while Republicans stayed home. The 2010 vote was the reverse.
But what about California? There's no way to definitively prove coattails, but the 2006 and 2010 gubernatorial races make an interesting contrast. In 2006 the Republican gubernatorial nominee, Arnold Schwarzenegger, got 55.9% of the vote. The 2010 nominee, Meg Whitman, got 40.9%. A wide spread like this one should produce the coattail effect, if it exists.
Below is a list of California congressional districts where there was one Republican and one Democratic candidate in both 2006 and 2010. So it includes 43 of the 53 state's congressional districts. The results are two party, so they include only the Republican and Democratic votes in both the gubernatorial and congressional elections. The chart can be found here.
Schwarzenegger beat Whitman by anywhere from 9.8% to 20.5% in the districts.In 15 districts the Republican did better in 2006, while the party's candidate did better in 28 in 2010. Overall, Schwarzenegger averaged 58.4% in the districts while Whitman averaged 42.7%. Yet the average Republican in 2006 got 40.9% of the vote and 42.7% in 2010. While you might not expect the average Republican to be 15.7% better in 2010, you'd expect them to be somewhat better if there coattails. You certainly wouldn't expect them to be 1.8% worse.
Some people might point out that Arnold Schwarzenegger wasn't closely identified with the Republican party and might have less of a coattail effect. That's true, but he'd been famous for nearly three decades and was well known as a Republican. He'd even spoken at the party convention. Whitman's ties to the Republican party were nonexistent. She hadn't been active in politics and few people knew what party she was registered with before she ran. She'd even rarely voted. If Neel Kashkari finishes top two, he'll have a similar non-identification with the party. Tim Donnelly is more closely identified with the Republican party, but he's not very well known.
Another factor that might come into play was the national environment. It favored Democrats in 2006 and Republicans in 2010, the reverse of the California gubernatorial result. That's true, but if that was a bigger factor in both 2006 and 2010, then it'll also be a bigger factor this year.
There is a Republican floor in California. Elizabeth Emken was an unknown, underfunded, candidate for the U.S. Senate in 2012 who ran against a well funded, extremely popular incumbent, Dianne Feinstein, in a Presidential year with a Democrat at the top of the ticket who got 61.2% of the two party vote. Emken got 37.5% of the vote. This election will be a mid-term more favorable to Republicans. Brown isn't as popular as Feinstein and the Republican gubernatorial nominee should have more funding than Emken.
If Kashkari or Donnelly are a disaster, they should get at least that 37.5% At that level the drop from Meg Whitman would be a fraction of what it was from Schwarzenegger to Whitman. If Republicans were able to do better with Whitman than Schwarzenegger I doubt Kashkari or Donnelly doing a few points worse than Whitman will drag down Republicans.