We all know that Dems are less likely to vote in midterms. Roughly it's younger people who are less likely to vote and those young people, especially where it matters, are more likely to be Democrats. After the 2010 midterms this somehow morphed into the idea that THE LEFT, as distinct from Democrats, chose not to vote because they were firebaggers, or immature purity politics peddlers, or burn-it-down-to-save-it types. This all came from an internet pie fight lens view of politics, where a few anguished Daily Kos Diarists somehow represented a giant movement of Leftists Against Obama who took their balls and went home and that's how we got Speaker Boehner.
I'm sure a few of those people exist, and not just online, but for the most part people who spend precious time actually thinking about politics, including ones writing anguished Daily Kos Diaries or pissed off wonky housing policy critiques on sucky baby blue blogs, actually vote. It isn't THE LEFT who doesn't vote in midterms, it's Democrats.
But it was a useful bit of blame shifting. It wasn't the people who spent hundreds of millions of contributor dollars (raised with increasingly disgusting fundraising tactics by the DCCC especially) and who recruited a bunch of crappy candidates who were at fault. No, it was the fault of the politically immature dirty fucking hippie part of the coalition. Especially the ones on the internet, the patchouli powered internet.
We're already seeing the general election version of this, which is "ZOMG BERNIE SANDERS SUPPORTERS WILL LOSE THE ELECTION FOR HILLARY CLINTON BY REFUSING TO VOTE." Good story bro, 10.5 months away from the election. Maybe there will even be some truth it! But it's the candidate's (and the campaign's and the party's) job to win elections. It's their job to get people to the polls and push the correct button. That's what they collected all that money and hired all of those people for. You have to win elections with the electorate you have, not the electorate you want.
I'd probably push the team 'D' lever in a presidential election (almost) no matter who the candidate was, and I'd probably argue with anyone who suggested that it's better for that candidate to lose than to win (which is the argument if you're arguing that people shouldn't vote). But when candidates lose, it isn't the fault of voters, it's the the fault of the candidates. Still, hey, go ahead and start blaming Bernie-bros 10.5 months away from the election. That's the clear path to victory! Or, at least, the clear path to excusing your own failure.