More evidence of this comes from polls suggesting that demographics do a better job than ideology of predicting vote choice. Self-reported ideology is an inexact measure. But Mr. Biden, for instance, has a far larger lead among black voters in most national surveys than he has among moderates. Ms. Warren and Pete Buttigieg, on the other hand, often have little support among black voters.

Of course, demographic splits can stem from issue positions. You could argue that President Trump’s stances on immigration and trade made him stronger among white working-class voters. And in a Democratic primary, you might think that the way to appeal to black voters is by emphasizing criminal justice reform, housing desegregation, the Black Lives Matter movement, reparations, and any number of other issues with particular and substantial appeal to black voters, based on polls.

But the issue-based explanation for demographic splits in primary politics often falls short. Hillary Clinton and Barack Obama, for instance, barely disagreed on the issues, yet they divided the electorate along stark demographic and regional lines. (The explanation is fairly obvious here: Mr. Obama was the first black major party nominee for president.)

Voters who are highly informed, ideologically consistent and politically active make up a minority of the electorate.

The rest — a group that commands the votes to decide the Democratic nomination — might prefer an emotional appeal to an analytical argument. They might default to well-known, battle-tested politicians whom they’ve grown fond of over the years. They might say they vote “for the person,” or “with their gut.” They might vote for Republicans on occasion. They might be drawn to superficial considerations like appearance and voice depth. And many voters might harbor traditional and even discriminatory notions of leadership that disadvantage nonwhite and female candidates.

When asked by a pollster, these voters might say they support Medicare for all. But they might not have thought much about it, and may not care much about the nuances that debate moderators try to tease out of the candidates.

All of this winds up posing a challenge to candidates who intend to appeal to voters based on policies reflecting a consistent worldview. The challenge is most obvious for the liberal, reformist and idealistic candidates who often manage to assemble a factional base of support among ideologically consistent voters, but usually fall short of the nomination.