Pete Buttigieg, the 34-year-old mayor of South Bend, Ind., is threatening to shake up the race for Democratic National Committee chairmanship precisely because he isn’t linked to either of the factions warring for control of the party.

He joins a contest that has turned into a proxy battle between the populist progressives, a group led by Sen. Bernie Sanders, I-Vt., that favors Rep. Keith Ellison, D-Minn., and the establishment wing of Democratic Party that is tied to President Obama and is lining up behind his labor secretary, Tom Perez.