Former Mexico City Mayor Andrés Manuel López Obrador is leading in the polls for Mexico’s July presidential election. Given his reputation as an antimarket extremist, this may seem surprising in modernizing Mexico. But he has been helped by the backing of Monterrey tycoon Alfonso Romo, whom the candidate has named chief of staff of his proposed cabinet.

The Romo endorsement aims to reframe López Obrador as a moderate who will befriend capitalists as long as they haven’t cheated to get ahead. Mr. Romo talks a good game. In...