Submitting this because I felt it had a good take on the question of electability that didn't go into the polling numbers - to quote the section that stood out to me (though all of it is a good read):

The reason to bet on Mr. Sanders being competitive in the general election has to do with a force beyond his control: the remarkable polarization of the electorate. More voters than ever despise the other party rather than embrace their own, a phenomenon known as “negative polarization,” and this means there are relatively few voters left who will be genuinely undecided between the Democratic and Republican candidates.

“The rise of negative partisanship means that very few Republicans or independents who lean toward the Republican Party will vote for Hillary Clinton or any other Democratic presidential candidate in 2016,” wrote Alan Abramowitz and Steven Webster, two political scientists at Emory University, last July. “Likewise, very few Democrats or independents who lean toward the Democratic Party will vote for … any of the other potential Republican candidates.”

Strongly partisan voting patterns will mean the likelihood of a very close election this fall, regardless of which candidates the parties nominate. Compared to presidential elections of decades’ past, there are a limited number of states that will actually be competitive for either party. In the 2000 election, 12 were decided by five points or less; that number shrunk to four in 2012. There is still little reason to think that the 2016 election won’t come down to the same few swing states. (Some have argued Mr. Trump, the billionaire real estate developer who has alienated minorities and the Republican establishment alike, could be the exception and face a rout.)

This is the best argument against the claim that Mr. Sanders, if nominated, will be another George McGovern, the ideologically pure South Dakota senator who carried a single state against Richard Nixon in 1972. In the 1960’s and 70’s, just about every large state was competitive. A Democrat could win Texas. A Republican could win New York and California. In 2016, it would be almost impossible for Mr. Sanders to lose major states that have backed Democrats over the past few cycles, alleviating the possibility of a McGovern-like landslide.