Twitter is often the intellectual equivalent of a tavern at 2 a.m., but it has illuminating moments. An example came Friday when Senator Elizabeth Warren, the Harvard populist, offered a hilarious commentary on her proudest political accomplishment—the Consumer Financial Protection Bureau.

“I’m giving @MickMulvaneyOMB one last chance to answer my questions about his actions at the @CFPB. If he won’t, he should be called immediately to testify under oath before my colleagues and me on the Senate Banking Committee,” the Senator...