The hot topic of conversation right now is the proposition that a long, drawn-out Democratic primary runs the risk of destroying the party and putting John McCain in the White House. So for the good of the country, Hillary should withdraw.
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.
But Drum fails to mention that one important reason for the closeness of the election was that George Wallace ran as a third-party candidate and won 13.5 percent of the popular vote. Back in 1969, political scientists Philip Converse, Warren Miller, Gerald Rusk, and Arthur Wolfe analyzed the data and showed that most of Wallace's votes would have otherwise gone to Nixon:
[T]he majority of his votes were from Democrats who otherwise preferred Nixon rather than from Republicans who might have given their favors to Humphrey. . . . all reasonable reconstructions of the popular vote as it might have stood without the Wallace candidacy leave Nixon either enjoying about the same proportion of the two-party vote that he actually won or a slightly greater share, depending on the region and the detailed assumptions made.
I think there are many reasons why 2008 hasn't been and won't be as bad for Democrats as 1968 was, but I'd be careful about using 1968 as an example that intra-party fights don't matter in general election outcomes.
Update: Emailing with Kevin Drum about this, it occurs to me that the better example might be 1976. In that year, Gerald Ford and Ronald Reagan fought each other all the way until the convention in August. Not only that, but the contest was pretty bitter. Ford accused Reagan of wanting to gut Social Security and of being reckless when it came to foreign policy. Reagan, for his part, accused Ford of being soft on the Soviets and for wanting to give away the Panama Canal. Nonetheless, Ford managed to close the gap with Jimmy Carter and only narrowly lost the election. Had Ford not freed Poland in one of the debates, he may well have won the election.