Once again someone drags out this tired old argument with the foregone conclusion that Hilary will without a doubt win the Democratic primary. This is not an argument for why progressives shouldn't support Bernie Sanders, it's an argument for why the most spineless of Democrats shouldn't. Progressives should support Bernie Sanders, because he's the more progressive candidate.
I'm a life-long Democrat. When it comes to general elections, I argue consistently that a third party vote is completely counterproductive. Mathematically, this is absolutely true and will be unless we completely overhaul our voting system. A vote for a third party candidate is functionally equivalent to a half-vote for the candidate you're less supportive of out of the two who have a chance to win.
Primaries are not general elections, though. Sure, we still use the same sort of voting, but the result of your candidate losing is still the most popular candidate from your party winning, so that's not horrible. Primaries aren't some sort of ideological trial by fire by which we test the purity of our Chosen Candidate against a gauntlet that poses no serious competition. Bernie Sanders has significant support. People are way more excited about him than they are about Hilary. Just because a lot of us have assumed for the past 8 years that Hilary would win the presidency after Obama doesn't mean it actually has to happen. Bernie Sanders sides with most voters on the majority of issues, including progressive voters who don't usually bother coming out for elections because the Democratic candidates aren't progressive enough for them.
Bernie Sanders isn't really a Democrat either. He's an independent who works with the Democrats, but he won't have the same sort of ties to party leadership guiding his actions. He's basically the closest thing to a third party candidate that we can have under the system we've got. He's the perfect progressive candidate.
It's not progressives who have a problem with Sanders, it's Clinton supporters. Don't let fear-mongering journalists cause you to vote against your own positions in the primaries. If you like Hilary, by all means vote for Hilary, but if you like Bernie, let's give him a chance! Just because Hilary supporters see him as little more than an obnoxious obstacle doesn't mean we should waver in our support for an electable candidate with significant buzz who would better reflect the politics of the entire country.