Will the Sanders-Clinton struggle lead to a gruesome fall defeat? History says no [View all]
but that's not the big question
http://www.salon.com/2016/04/09/party_unity_is_bogus_the_hillary_bernie_debate_is_crucial_and_wont_doom_the_democrats_in_the_fall/
As the current Democratic frontrunner cannot possibly have forgotten, Bill Clinton emerged from a brutal, mud-slinging primary campaign in 1992 and won easily in the fall. John F. Kennedy, Jimmy Carter and both George Bushes prevailed in memorably heated primary contests and then won narrow elections. (OK, only the elder Bush was actually elected: Oh, snap.) You can cite counter-examples of candidates who sailed through the primary season and won in November, like Ronald Reagan in 1980. But John Kerrys relatively smooth path to the Democratic nomination in 2004 didnt help much. Even the supposedly classic examples of parties ripping themselves apart with ideological infighting yield mixed results. Hubert Humphrey nearly won the 1968 election, despite not even having run in the primaries and garnering the Democratic nomination through backroom deals at the most disastrous convention ever. (Paul Ryan is reading up on this right now!) Barry Goldwater, on the other hand, vanquished the GOP establishment with the insurgent conservative crusade of 1964, and then got less than 39 percent of the popular vote, which remains the most lopsided defeat in history.
In short, there is no consistent pattern to suggest that a contentious primary campaign leads to electoral doom. The only pattern I see, in fact, is that the party-unity argument, and the threat that winter is coming, is consistently used as a club to bash dissidents. It seems wildly unlikely that the Democrats are heading toward any version of a Goldwater scenario in 2016, or even a Humphrey scenario. Which of our two parties faces that prospect, or something worse, is abundantly clear. But I think this entire line of argumentation undersells the symbolic and ideological significance of the Clinton-Sanders battle, which is exactly what the advocates of mandatory party unity are trying to deflect or deny or ignore.