I needed to come back to reiterate how dumb and stupid of an idea this is, and how angry I am that we're actually here on the left with a failed strategy that's plagued Republicans since 2008. In order to elect a working caucus, you need people who may not agree with you 100% of the time. There are 25 states that are blue states, or blue leaning states. There are 25 red or red leaning states. If we were to go by their COOK PVI, you'd get 50 Democrats and 50 Republicans (actually 49 Democrats and 51 Republicans since Virginia is EVEN, though I imagine that'll change after 2016 to be D+1).
This means that if you want to compete in the Senate in states such as Missouri, Indiana, North Carolina, etc, you're going to need to elect people who might not be the most progressive people on certain issue, but will vote for you a reasonable amount of the time. And especially when necessary. When push comes to shove, McCaskill will vote for her caucus. That's extremely important. McCaskill is not the most liberal person who has ever been elected to the Senate, but she is far more progressive than the average voter in Missouri, and still won (mostly because she's a shark and I'm sure she's probably killed a man if it needed to be done, but still).
You NEED people like this in your caucus if you want to attain enough votes to pass legislation, or hold a seat for 6 years when you might take a hit in a bad year in your states.
Even more importantly are governorships. John Bel Edwards, the recently elected governor of Louisiana is a conservative Democrat. He's pro-life. He's against gay marriage (this is more lip service, he helped stop a RFRA-type bill so I'd say he's good on the gays). He's pro-gun. Yet he's definitely on the left side of things economically. More importantly, he's the most liberal person you could reasonable expect to be elected statewide in Louisiana.
Does that mean you primary him, or any of these other people, from the left because he isn't as pure as you want? Fuck no.