It is very hard to point to differences between Mrs. Clinton and Mr. Sanders’s proposed policies that could plausibly account for such substantial cleavages. They are reflections of social identities, symbolic commitments and partisan loyalties.

Yet commentators who have been ready and willing to attribute Donald Trump’s success to anger, authoritarianism, or racism rather than policy issues have taken little note of the extent to which Mr. Sanders’s support is concentrated not among liberal ideologues but among disaffected white men.

More detailed evidence casts further doubt on the notion that support for Mr. Sanders reflects a shift to the left in the policy preferences of Democrats. In a survey conducted for the American National Election Studies in late January, supporters of Mr. Sanders were more pessimistic than Mrs. Clinton’s supporters about “opportunity in America today for the average person to get ahead” and more likely to say that economic inequality had increased.

However, they were less likely than Mrs. Clinton’s supporters to favor concrete policies that Mr. Sanders has offered as remedies for these ills, including a higher minimum wage, increasing government spending on health care and an expansion of government services financed by higher taxes. It is quite a stretch to view these people as the vanguard of a new, social-democratic-trending Democratic Party.

Mr. Sanders has drawn enthusiastic support from young people, a common pattern for outsider candidates. But here, too, the impression of ideological commitment is mostly illusory. While young Democrats in the January survey were more likely than those over age 35 to call themselves liberals, their ideological self-designations seem to have been much more lightly held, varying significantly when they were reinterviewed.

Moreover, warm views of Mr. Sanders increased the liberalism of young Democrats by as much as 1.5 points on the seven-point ideological scale. For many of them, liberal ideology seems to have been a short-term byproduct of enthusiasm for Mr. Sanders rather than a stable political conviction.

Perhaps for that reason, the generational difference in ideology seems not to have translated into more liberal positions on concrete policy issues — even on the specific issues championed by Mr. Sanders. For example, young Democrats were less likely than older Democrats to support increased government funding of health care, substantially less likely to favor a higher minimum wage and less likely to support expanding government services. Their distinctive liberalism is mostly a matter of adopting campaign labels, not policy preferences.