"The lack of a 'liberal Tea Party' reflects a fundamental and longstanding asymmetry between Republicans and Democrats."

"The Republican Party is the agent of an ideological movement; most Republican politicians, activists and voters view their party as existing to advance the conservative cause. Because their goals of reducing the scope of government and reversing cultural change are difficult to achieve in practice, Republican officeholders are vulnerable to accusations of failing to uphold principles..... The Democratic Party, by contrast, is organized as a coalition of social groups. Democratic voters tend to view politics as an arena of intergroup competition rather than a battlefield for opposing philosophies, and the party is dominated by an array of discrete interests that choose candidates on the basis of demographic representation and capacity to deliver policy.... This year, Democratic candidates remain focused on challenging vulnerable Republican-held seats more than purging ideologically impure incumbents.... Liberal candidates and activists can succeed in pushing the Democratic Party to the left on specific issues. But they will do so by appealing to the interests and loyalties of social groups rather than engaging in broader ideological debates."

Writes polisci prof Matt Grossman at "Why There Is No ‘Liberal Tea Party.’"

What he's saying about electoral politics rings true, and yet I also think it's true that it is the left that's looking for heretics and the right is looking for converts. That may not be a contradiction.

ADDED: I think I've figured out why it's not a contradiction. These are the 2 propositions: 1. The electoral success of the Democratic Party depends on maintaining a coalition of interests groups, and 2. Democratic Party partisans create pressure on each other not to get out of line. These ideas fit together well if you take their perspective: We need to stay united. It's not effective at winning converts, because there's no appealing idea to be understood and believed in. In fact, it's threatening to those who are outside of the group. These outsiders may insult the left by saying they are "looking for heretics," but the so-called "heretics" are only people in the group who undermine the adhesiveness of the group. The conservatives can "look for converts" because they do have some abstractions that seem appealing and coherent to ordinary people, and those people can feel burned when their representatives do various pragmatic things that deviate from the ideology.