Remember when Bill Clinton was twice elected President as a "New Democrat" who wanted to get along with people in the private economy? Nowadays even writing in favor of ideas that Mr. Clinton supported can get you branded a heretic and banished from impolite liberal society.

Witness the political tempest that blew up in response to the December 3 op-ed we published by Democrats Jon Cowan and Jim Kessler of the Third Way think tank. The former aides to Andrew Cuomo and Chuck Schumer, respectively, would be considered liberals in most places outside of the Harvard faculty lounge. But the campus and union left that increasingly controls the Democratic Party has launched a campaign to purge them for their deviation from progressive dogma.

Messrs. Cowan and Kessler had the temerity to suggest that the average American voter might not be as liberal as the New Yorkers who chose Mayor-elect Bill de Blasio or the Massachusetts voters who fell for Senator Elizabeth Warren. They argued that economic populism of the redistributionist kind was a political loser in most of America.

The authors were also guilty of acknowledging research from the Congressional Budget Office showing that Social Security is going broke. Progressives believe it's high treason to admit the undeniable fact that such entitlements are paying out more in benefits than they collect in taxes. If left unchecked, such candor might motivate politicians to consider reform, and the last thing the left wants is a debate over the limits of the welfare state.

But the progressives didn't try to rebut the op-ed's arguments. Instead they set out to silence Third Way, intimidate Democratic politicians and donors into disavowing the group, and discredit the think tank on grounds that—gasp!—some of its supporters work at financial companies.