Matt Grossmann and I explained in Asymmetric Politics why the Democrats are much less vulnerable to ideological purification campaigns than Republicans are, and we summarized our argument in this piece for Vox Polyarchy. Part of the story is that the American left simply lacks much of the institutional infrastructure that promoted and sustained the Tea Party rebellion on the right, such as powerful ideologically-driven media sources, interest groups, and financial donors. (The number of politically active leftist billionaires is....not large.) But it's also true that many Democratic voters simply don't think of politics in ideological terms or prize doctrinal fidelity over other qualities—such as perceived electability, group identity, or ability to deliver concrete policy achievements—when making their choice of candidate.
So if there isn't much evidence of a "liberal Tea Party," why is anybody talking about it? One reason is that the assumption of party symmetry is deeply entrenched in the minds of many political observers, who expect any trends on one partisan side to inevitably appear in comparable form on the other. Another is the well-documented tendency of media coverage to frame stories in ways that emphasize conflict, or at least the possibility of conflict ("if it bleeds, it leads"); for example, this recent Politico article does its best to hype the existence of a "Democratic civil war" exacerbated by Lamb's victory even though there's nothing in the actual piece that justifies using such hyperbolic language.
A third is that Republicans, facing a poor electoral climate this year, have adopted the talking point that their fortunes will be salvaged by a raft of extremist opponents nominated by far-left Democratic primary electorates. House Speaker Paul Ryan brushed off Lamb's victory last week by claiming that "this is something that you're not going to see repeated, because they didn't have a primary [referring to Lamb's selection by a local Democratic committee to compete in the special election]. They were able to pick a candidate who could run as a conservative."
But there's something else at work here as well. Purist leftism, to the extent it exists in America, is especially concentrated in the circles—metropolitan, well-educated, highly internet-active—in which many media members themselves travel. Based on their own anecdotal experiences, or at least their social media feeds, it's easy for them to start thinking that left-of-center politics is consumed with protests of ideologically unpalatable campus speakers, debates over whether Bruno Mars is guilty of cultural appropriation or whether RuPaul is prejudiced against the transgender community, and endless relitigation of the Hillary Clinton-Bernie Sanders presidential race as a proxy for the direction of the American left as a whole. (In reality, as my research shows, Clinton and Sanders supporters in 2016 were split much more by age, race, and party identification than they were by ideology.)
Put simply, the online left is not representative of the Democratic Party. Visitors to local Democratic caucus or committee meetings in most parts of America will find that the public employees, union officials, trial lawyers, nonprofit association administrators, and African-American church ladies who actually constitute the party's activist backbone are, by and large, neither preoccupied with ideological purity nor in a state of rebellion against its current leadership. And though the election of Donald Trump has surely angered and energized the Democratic base, there's no particular reason to think that anti-Trump sentiment will lead to an internal ideological transformation.
The scholars Lara Putnam and Theda Skocpol, who are studying the citizens—especially the women—newly mobilizing against the Trump-led GOP, report that a strong sense of pragmatism prevails among their subjects. "This is not a leftist Tea Party," they explain. "It is not a Sanders versus Clinton redux [or] Occupy Wall Street-type questioning of liberal democracy . . . [T]he metropolitan advocates to whom the national media turn . . . at times exaggerate the left-progressive focus of the activism underway and overestimate their own importance in coordinating it." Instead, Putnam and Skocpol find a lot of middle-aged suburban professionals moved to act by their horror of Trump and determined to work strategically to oppose him. "At the current pace," they predict, "it seems likely that the pop-up leaders and grassroots groups of 2017 will, by 2019, have repopulated the local layer of the Democratic Party in much of the country."
The logic of Asymmetric Politics doesn't imply that one party is inherently in better shape than the other, but rather that each side has its own distinctive set of problems. The Democratic Party is suffering from a number of contemporary weaknesses, made undeniable by its inability to defeat a deeply flawed Trump candidacy in 2016. But Democrats remain well-positioned to avoid the specific pathologies that have recently plagued the Republican opposition: endless primary challenges to veteran incumbents, Freedom Caucus-style legislative rebellions, the elevation of cable news hosts into positions of power over elected officials. Jettisoning the assumption that one party is simply a mirror image of the other would not sacrifice the balance and objectivity of news media coverage, but it would greatly improve its accuracy.