Bernie Sanders supporters: what should the medium term plan be?

…assuming Bernie comes in 2nd place.
Quick answer: vote every two years.

Longer answer: this question is impossibly broad. I’ll divide it into 2.

For social democrats, members of the US left fringe, and others desiring a country more in line with the rest of the advanced world

You already vote every 2 years. You want the US to attain strong economic growth rates and escape from Republican stagnation. Sweden for example outperformed the US in terms of real GDP from 2006-2014. (Sure, the US beat Sweden during the latter 2009-2014 era, but what do you expect? It was a Democratic Presidency.)

The key Republican challenge is to persuade the middle class to vote against its economic interests. The key progressive challenge is to flip purple states and purple districts wherever they are. Sanders widened the Overton Window this year and last year. So how do we build on that? I’m especially interested in the views of lefties residing in red and purple districts.
For other supporters of Sanders
The political system is rigged. Thanks to Citizens United and a pliable Supreme Court, pampered billionaires call the shots. Sure, sometimes they can be careless: they thought Jeb Bush would be a great candidate after all, even though he had no plan for distancing himself from the Iraq War. So much for vetting or thinking things through. This was a fail on both substantive and political grounds. Set aside Jeb’s flaccid political skills: his campaign strategy was half-baked. It’s almost if these funders suspend their business judgment and are simply enthralled when they brush shoulders with the politically well connected. I don’t know.

At any rate there are 2 goals. Replace Scalia, Thomas, Kennedy, Alito and Roberts with Supreme Court justices with more common sense: those clowns equated money with speech. Never mind that we basically have little idea of what the Koch Brothers et al actually think (why did they give Jeb $100 million?), except via their bizarre and misleading attack ads.

The second goal would be to take down the biggest opponents to campaign finance reform. Mitch McConnell was re-elected in 2014: he likes the status quo because it levels the playing field for those like himself who are bad on the stumps and lack personal charisma. Somehow we have to persuade his supporters in the polling booth that he and his Republican cohorts are completely on the take and really really like it that way. You can draw a direct line between their hands-off attitude towards Wall Street and their bulging campaign coffers.

So vote out the Republicans at the national level, even when a President isn’t running.

For myself
Frankly, I’d be happy if the Republicans turned back to the pragmatic and rationalistic interest group brokerage approach of Nixon/Ford/Eisenhower. But the only path to that destination involves the GOP’s complete electoral collapse, some reflection, a spell in the wilderness, and a reckoning. Nuke 'em from orbit.

Honestly, I’d be happy to see the “corporate” factions in both parties get wrecked.

I think the early play in the game has to be reorganization, to create what is in effect a new party that will fight for election reform, weakening the influence of gerrymandering.

Now, whether through entryism into existing state parties or a new third party is an open question. In my state, I think we could take over the Democrats, but it would be hard, as we’d have to basically declare war on the old conservatives in our own party. But it might be even harder to get a Bull Moose faction to take over the party of Gingrich.

Both parties. With respect, you are dreaming. Even with campaign finance reform, that won’t happen. Corporations have lots of money and many, many employees some of whom have sympathy for the suits.

And barring 1950s levels of unionization, the Democratic party would collapse without corporate funds. So it won’t happen. (Ok, it could happen with strong electoral reform, but that’s freaking unlikely as well). Luckily it doesn’t really need to happen. There’s plenty that can be done by merely overturning Citizens United in one way or another.
Let me be clear. Bear in mind that I don’t have any easy answers to the OP either. But the Sanders campaign is sort of silly without a Plan B, right? Because Sanders ain’t getting elected and if he does he ain’t getting his programs passed. For there to be progress, the Dems have to take additional seats in both Houses. Other than applying “More of the same,” (which can work) I can’t see a path forward.

Are Bernie supporters concentrated in blue districts? Do any of them in red or purple districts have any insight in getting their neighbors to flip to the Dems? Or getting youth to turn out in 2018 when there is no Presidential headliner?

I don’t know. It’s obvious to me after the mess that has been the Obama presidency that focussing on the White House is, well, idiotic. You can’t really pass any agenda if you’re not participating meaningfully in the parliamentary body.

I admit again: me neither.

Actually, the Obama Presidency has been extraordinarily successful. The 111th Congress was by far the most successful and accomplished in the post-war era, and they did this in the teeth of the most vicious partisan opposition in US history. It’s been painful to watch but achieving health care reform, Dodd-Frank, the stimulus package, the resurrection of the hunt for Bin Laden and his capture, the successful containment of Iran’s nuclear program, extraction from the Iraqi and Afghan quagmires, and curbs on greenhouse gases as an extension of the Clean Air Act is frankly more than we could have reasonably expected in 2008. The next President won’t have as good a track record.

People say that Obama is an historic President because of his race. I frankly perceive that as minor relative to his objective accomplishments.
But yes, somehow we need to translate this expanded Overton Window into a saner Congressional body. One not obsessed with shutting down the government and other dramatics.

BTW, anyone interested in this thread should read The Deep State: The Fall of the Constitution and the Rise of a Shadow Government, by Mike Lofgren. The 1% is one of our proper targets, but the Deep State is the other.