This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

John Boehner thought he had a clever plan. After failing to reach a compromise with the Obama administration to reduce the budget deficit and extend the George W. Bush tax cuts in December 2012, then-Speaker Boehner proceeded to what the House Republican leadership called "plan B." The House would vote to make the Bush tax cuts permanent for all but the highest-earning 0.2 percent of the population, putting Boehner in a strong bargaining position against his Democratic opponents as the clock wound down toward January 1, 2013.

If Congress and the president failed to act by that date, a simultaneous combination of tax increases and spending cuts would kick in automatically under existing law, jeopardizing the health of the national economy by sending federal policy over what official Washington dubbed a "fiscal cliff."

Unfortunately, this strategy ultimately resulted in a very public embarrassment for the speaker. Boehner was forced to pull the "plan B" legislation from the floor of the House after finding that it lacked the votes to pass due to significant opposition among his fellow Republicans, who, as the Washington Post reported, were unwilling to back the measure "because they worried it would lead to a primary challenge" from a hard-line conservative opponent.

Rep. Mike Coffman of Colorado agreed with this appraisal, telling the Hill that "there were members that are so gun shy about primaries that they weren't willing to take a risk" by backing legislation crafted by the leadership of their own party — even though the next congressional elections were nearly two years away.

It is difficult to imagine a circumstance in the contemporary Congress that would lead a significant share of Democratic members to publicly torpedo a strategy pursued by Nancy Pelosi because they were afraid of being denied renomination by angry left-wing primary voters. This telling difference reflects the fundamentally distinct character of each major party.

As we document in our new book, Asymmetric Politics: Ideological Republicans and Group Interest Democrats, the Republicans are oriented around a common ideology, while the Democrats are organized as a social group coalition. Internal disagreements within the Republican Party tend to take the form of accusations of disloyalty to the conservative cause, while intraparty battles on the Democratic side, when they occur, more commonly involve tensions among two or more groups under the party's big tent.

Republican challengers emphasize ideology; Democratic challengers represent group conflict

Since the 1970s, primary opponents challenging Republican members of Congress have been much more likely than Democratic challengers to attack the incumbent on the basis of ideology, as the figure below makes clear. Democratic challengers, in contrast, are more likely than Republicans to differ from the incumbent on the basis of social identity.

For example, the most competitive challenge to an incumbent Republican in the 2014 Senate primaries occurred in Mississippi, where six-term senator Thad Cochran narrowly prevailed in a runoff election over Chris McDaniel, a state legislator identified with the Tea Party movement who accused Cochran of failing to share "our solid, conservative values." The most competitive challenge to a sitting Democratic senator in 2014 came from then-Rep. Colleen Hanabusa of Hawaii, who nearly defeated incumbent Brian Schatz in a Democratic primary in which voters divided sharply along racial lines.

As the figure illustrates, the proportion of primary opponents who challenge Republican incumbents on the basis of insufficient ideological fidelity rose dramatically beginning in 2010, now constituting more than 40 percent of all contested races. Challengers to sitting Republican officeholders from the ideological right have not only become more frequent over time but have also come to represent a serious risk to their chances of renomination. Of the 31 Republican senators who sought reelection between 2010 and 2014, 10 — or 32 percent — were held to 60 percent of the vote or less in their home-state Republican primaries or renominating conventions, with three losing outright to more conservative challengers.

Today, a well-funded and influential network of conservative interest groups have joined with members of the Tea Party movement to form a powerful weapon of ideological enforcement aimed directly at the congressional Republican Party. Organizations such as the Club for Growth, FreedomWorks, and Heritage Action regularly intervene in Republican primaries on behalf of favored candidates. Support from conservative activists and groups allowed outsider candidates such as Ted Cruz, Rand Paul, Richard Mourdock, Sharron Angle, and Christine O'Donnell to win Republican primaries in recent years over "establishment"-identified opponents.

This trend has attracted the notice of Republican members of Congress—and frequently affected their behavior in office. When asked by a reporter why Boehner's "plan B" legislation failed to win support from the House Republican Conference, Boehner ally Mike Conaway of Texas replied, "You've got to ask Club for Growth [and] Heritage Action."

The introduction of legislative scorecards by conservative interest groups has also influenced the voting habits of congressional Republicans who wish to maintain quantitative evidence of their ideological loyalty in order to forestall electoral challenge.

Republican TV ads focus on the size of government; Democratic ads focus on groups and issues

The unique character of each party also extends to its communication with the public. As the figure below shows, we find that Republican congressional candidates are more likely than Democrats to mention ideological concerns about the general size and scope of government in their television advertising, though this rhetoric does not often extend to supporting the elimination of specific benefit programs that are popular with the public. Democratic candidates prefer to cite specific issues and groups rather than invoke broader ideological themes.

Sorry, Bernie: Democrats in the public don't want a more ideologically extreme party

The lack of preoccupation with ideological purity within the Democratic Party has allowed party leaders to avoid the threat of Tea Party–style primary challenges from their left flank. Democratic-aligned interest groups prioritizing concrete legislative action on behalf of their various group constituencies lack the motivation and capacity to enforce a strict ideological litmus test on the party's national leaders, and Democratic primary voters are less likely to be convinced that their elected representatives have forsaken their liberal principles.

As the figure below demonstrates, most Democrats in the mass public actually believe that their party should move further to the ideological center rather than the left.

In the wake of Bernie Sanders's unexpectedly strong second-place finish in the 2016 presidential primaries, some commentators suggested that Sanders and his allies would work to build an organization intended to further the cause of purist liberalism within the Democratic Party. But it is unlikely that a counterpart to the Tea Party would find comparable success on the American left, because the Democrats and Republicans are fundamentally different types of parties.

Any movement that threatens to sacrifice electoral pragmatism and discourage acts of political compromise in favor of strict adherence to a set of abstract values must contend with the fact that the social groups that constitute the Democratic coalition prize symbolic acts of ideological devotion less than the delivery of concrete policy change.

Matt Grossmann is the director of the Institute for Public Policy and Social Research at Michigan State University. David A. Hopkins is an assistant professor of political science at Boston College. They are the authors of Asymmetric Politics: Ideological Republicans and Group Interest Democrats.