Mr. Kuhn notes that Ms. Warren has a powerful appeal among the party’s activist left. Her rhetoric is pure firebrand: “The game is rigged . . . and the rich and powerful have lobbyists and lawyers and plenty of friends in Congress.” She draws big, enthusiastic crowds. Mr. Kuhn quotes Democratic campaign veteran Joe Trippi, who suggests things may be more dynamic than they look: “The progressive wing is looking for a candidate.” With Hillary, as they say, Democrats are falling in line but not in love.

Yes, Mrs. Clinton is the favorite; yes, she has the money, the clout, the stature, fame and relationships. But she’s no populist, and populism is rising. Hillary is close to Wall Street; they’re her friends, her donors, they hire her for big ticket appearances. Ms. Warren has no use for Wall Street; they’re the ones who crashed the economy and got away with it. Mr. Kuhn notes that Ms. Warren’s signature line—the game is rigged—is no longer radical; it is the view of 6 in 10 Americans in some polls that our economic system unfairly favors the wealthy.

Ms. Warren would also take Hillary’s most powerful argument—that it’s time for a woman president and she is an accomplished woman—off the table.