In particular, the two annoying suppositions that Sanders would've beaten Trump and that Kasich would've done much better against Hillary than Trump did.
It is very unlikely that a loser in the primary would've done better in the GE than the eventual nominee, and there is one big reason why this is true. The fact is that it is much, MUCH harder to win a primary than it is to win a GE. You have to battle members of your own party, which requires much more nuance than battling the sworn enemy. Also, a primary is rarely a binary contest, which means you have to siphon votes from multiple challengers. Finally, primaries are much more complicated than GE's, and require a lot more strategy in terms of timing and delegate math.
Winning a presidential primary in and of itself puts you on a much higher plane than any of your challengers, and it is foolish to think that an also-ran in such a contest would have the gumption to defeat the nominee on the other side.
Would Ed Muskie or HHH have lost 49 states in 1972?
Would Fritz Hollings or John Glenn have lost 49 states in 1984?
Would the 1988 Al Gore have carried at least SOME Southern states and been more competitive in 1988?
Presidential primaries are ideologically driven. Presidential General elections are driven by the ability to win a small number of folks who are moderate swing voters, while holding on to your base. The primary process also does not need to be won with a majority of votes, while the General Election requires someone approaching a majority (high forties).
Sanders, I would say, is an unusual case. His polling was what it was, and his appeal to certain white working class voters in primaries was remarkable. He appealed to these voters in areas where the left and right intersected (trade, foreign involvements, big corporate bailouts). The problem was that Donald Trump was the nominee, and many of those Sanders voters were folks for whom Trump would have been their favorite Republican, and they would have found themselves in greater agreement with the GOP on social issues. A similar argument was made in 1972 when there was shown a degree of convergance on some issues between McGovern and Wallace primary voters. This, of course, vanished quickly; Wallace Democrats voted heavily for Nixon in 1972, North and South.
