Roll Call is the latest with the story that "Donnelly could keep moderate Republicans home, and motivate Democrats to head to the polls."
The article reads like a Kashkari pitch, not a careful analysis.
It's certainly possible that some moderate Republicans won't want to vote for Donnelly. But if these people stay home, then that's on the Republican GOTV effort in each district. If Carl DeMaio can't get Republicans to the polls who have already decided to vote for him, then he's failing. They can always leave the governor box blank or vote Jerry Brown if they prefer him. I've been involved with Republican GOTV efforts the last few weeks and I have yet to hear any Republican voters say a bad word about Donnelly, let alone tell me they won't vote because he's on the ballot.
Donnelly could motivate Democrats to the polls, but the people most fired up about Donnelly are voting anyway. And there are a few impediments to this Democratic motivation theory. Donnelly wouldn't have a chance at winning. It's questionable whether he has a chance to finish 20% behind Brown. Can you really motivate people to prevent Donnelly from winning when he has no chance of winning?
Jerry Brown isn't going to run anti-Donnelly ads. His campaign is all about the positive. Like Dianne Feinstein he's not even going to mention his opponent, because he's running above politics. Running anti-Donnelly could backfire on Brown. It might motivate some on the left, but Republican bashing could really turn off Republicans who support him. He could turn those votes into blank ballots and lose more votes than he'd gain by Donnelly bashing.
It'd be up to outside liberal Super PACs to exploit Donnelly. They may successfully tie Carl DeMaio to Donnelly, but it'll just be substituting one villain for another. Right now they're tying DeMaio to the Koch brothers. It's not like they'll lack for villains. The Super PACs will tie any Republican candidate to big oil, obstructionist Republicans, and maybe they'll even revive George W. Bush.
One thing everybody is missing is how motivated Donnelly's supporters are. They have grassroots groups all over the state and they'll be out to support Donnelly. They're unlikely to be active to support Kashkari. I've been at two events where Kashkari failed to inspire. Most of these people are likely to vote anyway, but it's no more unrealistic to expect some Tea Party voters to stay home for Kashkari than moderates for Donnelly. At the very least Donnelly will motivate voters to the polls. Kashkari won't do that. His biggest asset is that he won't turn them off to stay home.
Edit: Over the last few years the conventional wisdom I heard at California Republican conventions was that Meg Whitman's weak candidacy compared to the Governator hurt Republicans down ballot. No one bothered to find out that Republicans down ballot did better in 2010 than 2006. They didn't need to. They knew Republicans did worse.