The predictability of election outcomes from fundamental variables suggests that different presidential candidates from the same party don’t differ much in the votes they will receive in the general election. It’s better to be a moderate than an extremist, and it’s better to be a better campaigner etc., but all these things together probably only count for a couple of percentage points of the vote.
Steven Rosenstone wrote about this in 1984 in his book, Forecasting Presidential Elections, and I don’t think the elections since then have given any reason to doubt Rosenstone’s logic.
Now, don’t get me wrong: a couple of percentage points of the vote can make a big difference–just look at the tied elections of 1960, 1968, 1976, and 2000, as well as the very close election of 2004. Also, who knows how things will go with the unprecedented “woman or young black guy vs. old white guy” dynamic. But, based on past elections, I’d say the whole “electability” thing is overrated. Once Election Day comes around, people will find a reason to vote for the party they want to support.