The first is exemplified by the candidacy of Bernie Sanders, the Vermont independent-socialist senator seeking the Democratic presidential nomination.

Sanders is running on an explicitly left-populist platform. It includes taxation of overseas corporate profits, a progressive estate tax, an increase in the minimum wage to $15 an hour by 2020, the investment of $1 trillion in infrastructure, withdrawal from Nafta and other trade agreements, free tuition at public colleges, a single-payer health care system, and more.

The problem is that the core of Sanders’s support, according to an October 2 Pew Research Center survey, is more concentrated among the college-educated than among those without degrees, and stronger among middle-class and affluent Democrats than among low-income Democrats. For now his messages appear to have caught on primarily among ideologically liberal voters, although there is an argument that it will resonate with others as they learn more about it.

Most important, in recent years, the Democratic Party has become the political home for those whose most passionate cause is cultural, as opposed to economic, liberalism: decriminalization of drug possession; women’s rights; the rights of criminal defendants; and rights associated with the sexual revolution, including transgender rights, the right to contraception, abortion and same-sex marriage.