Thus, while it’s true that fewer Americans self-identify as liberal than as moderate or conservative, this tells us almost nothing about voters’ policy views. “Moderates” do not actually display a preference for “centrist” positions, but merely for ideologically inconsistent ones. In fact, the Stanford political scientist David Broockman has shown that moderates are just as likely to subscribe to “extreme” policy positions as other voters are: In the United States, there are self-identified “moderates” who support a $1 million maximum income, prohibiting gays and lesbians from teaching public school and the mass deportation of all undocumented immigrants.

Meanwhile, the number of genuine “liberals” and “conservatives” is far smaller than meets the eye. Most voters who identify with those terms are partisans first, and ideologues second. Or as Mr. Kinder and Mr. Kalmoe conclude an analysis of four decades of voter survey data, “ideological identification seems more a reflection of political decisions than a cause.” In other words: The average conservative Republican isn’t a Republican because she’s a conservative — she self-identifies as a conservative because she’s a Republican.

One crucial implication of this finding is that political elites have enormous power to dictate ideological terms to their rank-and-file supporters. For a healthy chunk of conservative Republicans and liberal Democrats, the “liberal” and “conservative” position on most issues is whatever their party leaders say it is. Donald Trump’s success at redefining conservative voters’ consensus views on free trade, American policy toward Russia and the relevance of personal morality to effective political leadership offers a particularly vivid illustration of this phenomenon.

When we look past ideological self-identification to polling on discrete public policy questions, America appears to be far more center-left than center-right. In a recent analysis of Democracy Fund Voter Study Group survey data, the political scientist Lee Drutman found that 73.5 percent of the 2016 electorate espoused broadly left-of-center views on economic policy.

That finding is supported by polling on individual fiscal issues over the past year. Recent surveys have shown that most Americans — including majorities of Republican voters — support increasing federal financing of health care and oppose cutting taxes for the wealthy. And there’s little evidence that the Democrats’ left flank is exhausting the public’s tolerance for government intervention in the economy: Recent polls have found that over 60 percent of Americans support tuition-free public college (a majority that includes 58 percent of independents and 47 percent of Republicans); that over 60 percent of all voters favor Medicaid and Medicare buy-in programs, while a slim majority likes the sound of single-payer; and that 82 percent of voters, including 70 percent of Republicans, support new legislation expanding access to paid family and medical leave.

The Democratic Party has failed to translate the popularity of progressive economics into electoral success for a variety of reasons. The most fundamental is the one we’ve already observed: Most voters cast their ballots on the basis of identity, not policy. And America’s rapidly changing demographics — and the right’s steadfast efforts to inflame and exploit anxieties about those changes — have made racial identity increasingly salient to white voters, particularly rural ones. This development, combined with the disproportionate influence that our political system awards to white rural voters, has given Republicans a structural advantage.

Democrats have all kinds of ways of addressing this problem. One would be to cultivate the class identity of white voters by embracing populist rhetoric that paints “the billionaire class” as an out-group they can define themselves against. Another would be to invest more resources into registering nonwhite voters. According to the Census Bureau, 74 percent of non-Hispanic whites are registered to vote in the United States. For African-Americans, that figure is 69 percent; for Hispanics, 57; and for Asian-Americans, 56. As the policy analyst Sean McElwee has noted, this means that even if every registered Latino voter in America went to the polls last November, Latinos’ overall turnout rate still would have been lower than it was for whites (63 percent).

Embracing a more conservative economic agenda, however, would solve none of the Democrats’ problems. At a time of historic inequality, rampant corporate consolidation and environmental crisis, the case for more robust redistributive social programs and public-interest regulations is strong. When centrist Democrats claim that making such a case is electoral suicide, they reveal less about the American public’s stubbornly center-right convictions than about their own.