Now, this might be true. But 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.
If long, bitter, primary campaigns really destroy parties, then Humphrey should have lost the 1968 election by about 50 points. "Bitter" isn't even within an order of magnitude of describing what happened that year. And yet, even against that blood-soaked background, Humphrey barely lost.
So I say: chill out. Like a lot of people, I'm not very happy about the direction the Democratic campaign has taken, but the idea that it's going to wreck the eventual winner's chances in the fall seems pretty far fetched. It takes more than a few nasty exchanges to do that. And who knows? By keeping Dems in the spotlight, it might even help them. Stranger things have happened.