Interesting. I've never heard anyone call the Reformed Church left-wing. Historically, the Magisterial Reformers (Lutherans, Reformed) were the center between traditionalist Catholics on the right and the Radical Reformation (Anabaptists) on the left. In his book Interpreting John Calivin, Ford Lewis Battles paints Calvin as a radical centrist, posed between Catholic-leaning Luther and Anabaptist-leaning Zwingli on the topic of sacramental theology.
I've never heard the Reformed referred to as left-wing, but I suppose the term is relative.