For the "Bernie can't win the independents" crowd:
This piece on his popularity with independents -
"Among independents [in Iowa], you, Bernie, were the runaway favorite--with 69 percent of the nonaligned voting for you, compared to 26 percent for Hillary, almost a 3 to 1 margin."
For the "Moderate/conservative democrats won't support Bernie" crowd:
This piece on his popularity with those groups.
For the "Conservatives will oppose Sanders even more fiercely than Clinton" crowd:
This piece on Sanders' intention to court Trump supporters; and
This piece on Trump's very unconservative economic populism; and
This piece on Clinton's unfavorability numbers relative to Sanders across all voters.
So: (1) data from independents; (2) data from moderate/conservative democrats; (3) strong arguments on where the substantial anti-establishment sector of the right will turn if (the, relatively speaking, economically liberal) Trump is eliminated; and (4) deeply entrenched negative views of Clinton relative to Sanders across the political spectrum point to the conclusion that Sanders is more electable than Clinton.
Now, to address the opposing argument from Vox and their 6 political scientists -
I was genuinely shocked by how poorly reasoned this is. The idea that the Republican nominee could successfully portray Sanders' health plan as a threat to what people already know is downright hilarious. What people already know at this point is Obamacare - and I would love, love, in a way I cannot express, to see the Republican campaign on "Vote Sanders and you might lose your Obamacare!" - and Medicare - which is why the Sanders plan is branded and marketed as Medicare for all. In fact, the loss aversion argument clearly goes the other way: it has now been long enough for a very large segment of the American people to grow accustomed to the actual benefits of Obamacare, and Republicans have dug themselves a hole much too deep to get out of by their implacable opposition to it in the absence of any proposal for a replacement or alternative. Whatever the public perception of the ACA, and whatever the significance of that perception - however distorted or misinformed it might be - in general, the Republicans have no "loss aversion" argument available to them, since their entire healthcare platform is based precisely on a promise to take something away which people currently have, and it is much too late, and they have much too much invested, for them to turn it into anything else at this point. That is true regardless of how many people recognize that what they now have is good for them compared to what they had in the past, or not. That point should be perfectly obvious.
The unstated assumptions in the rest of the piece are that 1) Rubio will be the nominee rather than Trump or Cruz (both of whom Sanders would crush); and 2) Rubio is a moderate like Romney. Maybe Rubio will be the nominee; but 2) is, again, laughable. He's slightly to the left of Cruz and Trump, yes, but he's a Tea Party candidate and incredibly extreme in absolute terms. In the general, he would in fact face the mirror image of Romney's devastating problem: Romney was a moderate who had to try to pass as an unhinged right-wing lunatic, but failed. The failure meant he didn't get the lunatic vote, but the attempt meant he lost moderate votes. Rubio is an unhinged right-wing lunatic who has to try to pass as a moderate. Since Democrats will be turning out in huge numbers in the general no matter who the nominee is, a Rubio victory would have to rely on a move to the center which wins over enough moderates/independents to offset the hardliners (the ones who would be more comfortable with Cruz) who will end up feeling betrayed by that move, and staying home.
Essential to that would be moving back to the center on immigration, which he can't easily do, because he has so publicly and enthusiastically abandoned his centrist position on that issue to be competitive in the primary. And the current and ephemeral illusion - because that's what it is - that he's already pretty close to being as moderate as he needs to be (while also being sufficiently conservative) is based solely on (1) not being Trump/Cruz; and (2) being the pick of big business and the Party establishment. (1) disappears in the general, and (2) doesn't help him that much with moderates/independents this year. That aside, the idea has to be that over the entire course of the general, the Democrats wouldn't be able to scare independents off him based on the rest of his actual, quite extreme, record. Not even I think they're that incompetent. Rubio can't win the general without moving to the center, and he can't win the general without not moving to the center. In other words, the Republicans have built themselves a primary system which currently makes it impossible for them to win the general.
The snag in that story is that if the Democrats nominate Clinton, Republican voters will go to the polls no matter who their own nominee is. Clinton would have to make that up with a big share of moderates/independents, and I'm not at all convinced that she would end up doing better with them than Sanders, because 1) she has major issues of trustworthiness with them; and 2) Sanders' leftism isn't hurting him with moderates/independents on anything like the scale many seemed to be anticipating in this election cycle. Sanders, moreover, can call himself a socialist, or whatever else he wants, and still won't need as many moderates/independents as Clinton because the vast majority of this country's die-hard conservatives won't be moved against him to the same extent that they will be against her. There is no one they could hate as much as they hate Hillary Clinton, and one of the most significant things we have learned about a sizeable portion of this group in this election year is precisely that they are anti-establishmentarians first, and evangelical Christians/rugged individualists/free-market disciples second. Sanders would actually pick up some of the anti-establishment vote in the absence of Trump - his radically different policy positions won't matter nearly so much as the fact that he's the only other candidate not taking big money and challenging the economic and political establishment head on - and in the presence of Trump, he wins easily.