The Bernie Sanders primary campaign should be a lesson for progressives.
There have been progressives active in this country for just about forever. It should be a unified movement, but it's actually several movements, each with a fairly narrow set of beliefs. The progressive "movement" is made up of many individual groups, from environmentalists and anarcho-syndicalists to animal rights activists and other more or less single-issue-focused groups.
That's been going on for years. Normally, progressives don't field a presidential candidate with any hint of viability. This year, Bernie gave them a chance to come together and try an end run for the White House. But, they've always been around, and often try to get people elected to legislative offices. Typically, that's less than successful, but most congressional and state legislative races have progressive candidates running, right through the primary. Those candidates could be more successful, if only people understood how to make them successful. It's actually pretty simple.
The problem is that in most areas, there just aren't enough progressives who actually work together to overcome the Democratic Party's endorsed incumbent or candidate. It's easy to run for office in most states. Getting past the primary, on the other hand, is usually very difficult, indeed, if there is a solid Democrat running who can get endorsed by the local Democratic Party organization.
It would be easier if more people bothered to show up to primaries where legislative offices were being contested, but we're not very good about doing that, really. That's especially true in mid-term elections, unfortunately. So, in Democratic legislative districts, it's usually a well-known, long-time Democrat who gets the nod in the primary.
If you want to know why that is, volunteer as a poll worker in a mid-term legislative primary election and watch who is coming through the door to vote. You'll see the same faces at every primary election. They're almost all long-time democrats who vote in all elections, reliably and out of a sense of responsibility to participate. They're the actual base of the Democratic Party - the voters who can always be counted on to come to the polling place, no matter what offices are up for grabs. They even show up for odd-year city and county elections. When they do, they vote for the Democrats they know and skip the ones they don't.
And there you have it. If progressives want to elect more progressive legislators, they're going to have to do the hard work of bringing enough enough voters together who agree with them and get them to those under-attended primaries. It should be easy to do, given the low turnout for primaries, but it rarely seems to happen, really. The solid, reliable base, though, does turn out and vote for their solid, reliable candidates. That's why they're treated as the base.
Don't tell anyone, but I just explained how to have a real voter revolution in this country. It's a secret, though, so don't spread the word around. Thanks.
Note: This material in this OP was posted in another thread as a reply. Some people suggested that I make it an OP. As always, this is my opinion. Thanks for reading it.
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I helped them get elected. 
I may very well take you up on your offer tomorrow, or the day after. Or perhaps I won't bother ever again. We'll see how I feel.
I don't know you, so obviously, it can't be a personal thing.
Cheers. 
You can't fight City Hall, so burn the motherfucker down. 
Telling 'progressives' how to win - you're tired of the whining, too?
