Drum says for everyone to "chill out":
I'd like to offer a historical counterexample: 1968. Consider. The Democratic incumbent president was forced to withdraw after a primary debacle in New Hampshire. The Vietnam War had split liberals into warring factions and urban riots had shattered the LBJ's Great Society legacy. A frenzied primary season reached all the way to California in June, culminating in the assassination of Robert F. Kennedy. The Democratic Convention in Chicago was a nationally televised battle zone. Hubert Humphrey, the party's eventual nominee, had never won a primary and was loathed by a significant chunk of the liberal community. New Left radicals hated mainstream Democrats more than they hated Republicans.
In other words, this was the mother of all ugly, party-destroying campaigns. No other primary campaign in recent memory from either party has come within a million light years of being as fratricidal and ruinous. But what happened? In the end, Humphrey lost the popular vote to Nixon by less than 1%. A swing of about a hundred thousand votes in California would have thrown the election into the House of Representatives.
This is all true, but it's important to remember that Humphrey drastically underperformed in 1968 relative to what we would expect given the state of the economy at the time (a result that is often attributed to Vietnam War deaths). We can't quantify what damage was done by the polarizing primary campaign, but it's hard to see how it would help.
Democrats risk a similar scenario -- a destructive primary campaign could turn a possible rout in November into a 50/50 coin flip a la 1968. And even if they do win, any significant reduction in the popular/electoral vote margin could have significant legislative and political consequences going forward.