But when we examined exit poll data from the 2016 Democratic primaries, that’s not what we found. Ideology was not the most important dividing line separating Clinton’s and Sanders’s supporters. In fact, Clinton won 53 percent of the vote among self-identified liberals. She even matched Sanders’s support among the 25 percent of Democratic voters who described themselves as “very liberal.” At the same time, a full 36 percent of Democrats who identified as moderates or conservatives supported Sanders, a self-proclaimed “democratic socialist,” over Clinton.

But if ideology wasn’t what divided the Democrats, what was?

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Identity.

Most Democrats chose between Clinton and Sanders because of group identity, not ideology. Sanders performed very well among young whites and political independents. But Clinton received more votes in the end because she won the strong support of several key social groups within the party, including African Americans, Latinos and women. Instead of building her campaign around a single ideological theme, Clinton offered an array of policy proposals designed to appeal to a variety of specific constituencies.

We suggested in our post yesterday that Republicans and Democrats conceptualize their parties quite differently. In our new book, “Asymmetric Politics: Ideological Republicans and Group Interest Democrats,” we argue that Clinton’s winning approach reflects how the Democratic Party is a coalition of discrete social groups. The Republican Party, in contrast, is the vehicle of an ideological movement.

Our argument builds on the perceptive analysis of Jo Freeman, a political scientist with rough-and-tumble experience in party and interest-group politics, who described the two parties’ distinctive cultures in a seminal 1986 article. “The Republican party is not a poor imitation of a normal coalition-building party” like the Democrats, she observed, “but a different type of political organization that does things in different ways.”

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This year’s presidential election confirms the ways in which the Republican and Democratic parties are very different animals. But scholars of American parties — as well as many journalists — have recognized this only slowly.

“Political parties” are not all alike. They’re not even necessarily comparable.

Political scientists have developed theories of political parties intended to apply equally to Republicans and Democrats, without acknowledging how different in character the parties are.

For example, several scholars in the political science department at UCLA— some of them best known for the argument that party elites decide presidential nominations — recently introduced a new theory of political parties. This theory argues that Democrats and Republicans are really rival coalitions of “policy demanders” — groups that seek political power to implement their favored policies. According to the UCLA theory, this would better account for the extreme party polarization that we see today in the United States. They further argue that political ideology is really just a way of rationalizing the collective interests of each party’s most powerful constituent groups — and that it’s not a coherent set of abstract beliefs to which leaders, activists and citizens sincerely subscribe.

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That’s true enough — at least for the Democrats. But it’s not so true for the Republicans, who have organized their own party to advance an ideological cause defined by a set of common values.

The distinct behavior of each party’s candidates reflects this asymmetry. Democratic politicians pay close attention to what policies key constituent groups demand. These groups maintain separate and highly visible identities — even though their group leaders are conscious of being components in the larger party network (examples include labor unions, racial and religious minorities, feminists, gays and lesbians, and environmentalists). The Democratic policy program can be characterized as the aggregation of what these groups want. Democrats sell those benefits directly, without wrapping them in a cloak of ideological values. The social groups that make up the Democrats’ organizational and activist network bind individual voters to the party.

Republicans, rather than bargaining or competing with each other over clashing policy demands stemming from distinct group priorities, instead differ internally over the proper definition and application of the conservative doctrine. They all agree that that’s what the party should stand for — but disagree over which party members are, and are not, true conservatives. They show less interest in policy details or execution than they do in upholding the symbolic ideals of limited government, American nationalism and cultural traditionalism.

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When the two parties compete in general elections, Democrats benefit if voters see the election as a referendum on particular policy programs or as an opportunity to influence which particular groups are represented in government. Republicans, on the other hand, benefit when voters see elections as contests between liberal and conservative values.

We illustrate this difference by examining each party’s campaign rhetoric. Consider the graph below, which compares the rhetoric in Democratic and Republican presidential nomination acceptance speeches at party conventions since 1948. The patterns reveal that Republicans are far more likely to invoke ideological principles and far less likely to promote policies. Democrats are most likely to invoke particular groups they want to represent or oppose.

These differences also emerge within each party’s extended networks. We find that, during the primaries for congressional and presidential nominees, Democratic politicians and their campaigns more actively promote group membership and endorsements in their communications with voters. Republican congressional incumbents are much more likely to be challenged by ideologically extreme primary opponents. Democratic convention delegates, donors and activists are more likely to view themselves as representing specific interest groups than their Republican counterparts. Republican party organizational leaders report that they are more concerned with ideology in candidate recruitment. Democratic Party organizations and interest groups share more common ties, as measured by experiments measuring the use of organizational mailing lists.

These differences even extend to policymaking. Democrats in Congress compromise internally to pursue the adoption of a wide range of policies sought by the party’s constituent groups. Republican officeholders are more concerned with exhibiting ideological purity than legislative productivity. Democratic legislators also are active in many more caucuses focused on particular issues or constituency groups, such as the Congressional Black Caucus and the New Democrat Coalition.

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That’s why no single theory of party organization can accurately define both U.S. parties. They’re organized differently; they appeal to voters differently; they nominate candidates and seek policies differently. The Democratic Party is a coalition of groups that pool their collective political power to pursue a set of separable interests and priorities. The ideologically oriented Republicans are another type of party altogether.

Matt Grossmann is director of the Institute for Public Policy and Social Research and associate professor of political science at Michigan State University. Find him on Twitter @mattgrossmann.

David A. Hopkins is assistant professor of political science at Boston College and blogs about U.S. politics at Honest Graft.