Those that don't adhere to progressive orthodoxy are pariahs to the left. The progressives worked so hard, spending $10 million, to beat Blanche Lincoln in Arkansas in 2010. Lincoln was a long shot to win, so Democrats wanted to teach any candidate who strayed a lesson. Republicans won the Arkansas senate seat in 2010.
They are now going after Bob Kerrey, the only person who gives them a shot at winning a seat in Nebraska. Make life easier for the GOP
Alan Lowenthal, the Democratic candidate for CA-47, adheres closely to liberal orthodoxy. He has, however, dared to ask questions about the bullet train project. Horrible ones, like whether we'll have the money for it, if it's the best way to solve the problem, and whether it'd be better to spend the money on schools like the evil GOP wants. He isn't completely against the bullet train. He just thinks the current plan is unworkable.
If we're lucky, Democrats will punish him for such thoughts and make sure he loses. I hope they use Lowenthal as an example of what'll happen to Democrats who "side with Tea Party Republicans"