From a moral perspective this approach should make you think less of Biden, since it suggests he regards his principles (religiously-informed ones, supposedly, in the case of his abortion votes) as ultimately dispensable, but his Irish-uncle shtick as too essential to repudiate.

But the approach has a certain strategic logic. It assumes that activists can be appeased with specific promises, while the moderation of many older Democrats manifests itself more in general cultural attitudes (the kids these days with their political correctness, grumble grumble) than detailed policy preferences. It assumes that absorbing a certain kind of attack from the left — Biden’s too nostalgic for the bad old days, Biden’s a chump if he thinks he can cut deals with Republicans — helps with voters who are nostalgic for the days of dealmaking themselves, without making it impossible to eventually unite the party in the way that staking out heretical positions might.

These assumptions are by no means crazy. But it is also very easy to see how they might fail. Biden is hardly the most formidable of front-runners, and if any piece of the current coalition breaks off, he’ll be in deep trouble. And his rivals have obvious plays that might make that breakup happen.

The first play is to split off some of Biden’s African-American support by linking his nostalgia for dealmaking to his less-than-progressive record on race. This is the play Cory Booker is trying to execute, and Booker’s campaign probably depends on its success — which is by no means foreordained, since the African-American vote is more conservative and Biden-friendly than the median white liberal. But nobody before his years as Obama’s V.P. would have regarded Biden as a natural destination for the black vote, and enough sustained tone-deafness in the present could make that past matter once again.

The second play is to make the broader “Uncle Joe” persona, not just its unwoke element, a liability for Biden’s elect-me-to-beat-Trump case. This is probably the Pete Buttigieg play, though it’s available to any younger candidate: Without saying so directly, establish contrasts that make Biden look old, confused, a man out of time, even Trump-y in his own right. Don’t challenge his record, don’t call him a troglodyte — just challenge the conceit that a likable but past-his-prime uncle makes the right contrast with Trump.