In its first century and a half, the Supreme Court never used the term “federalism” in its opinions. The Court had talked about federal-state relations before, but the concept had gone unlabeled. That changed in 1939. Something new was happening, thanks in large part to Justice Felix Frankfurter. Just a month after joining the Court, Frankfurter authored the Court’s first opinion using the term “federalism.” Frankfurter introduced federalism as a key concept for analyzing the relationship between state courts and federal courts. Before long, Frankfurter would rely on federalism to fashion an original and enduring doctrine of judicial federalism: abstention, which requires federal courts to sometimes refrain from hearing cases that are within their jurisdiction.

This Article provides a historical study of Frankfurter’s contribution to the modern law of judicial federalism. It documents Frankfurter’s theory of federalism in his judicial opinions with a focus on the abstention cases. It also shows how the abstention cases and their concept of federalism were rooted in Frankfurter’s Progressive politics. They were a reaction to what he perceived as the federal courts’ anti-regulatory and anti-labor attitudes.

The history—relevant today as the political discussion around the courts again echoes the Progressive Era—sets the stage for considering the future of abstention. I suggest three possibilities. The first, an originalist future, would more or less maintain the contemporary Supreme Court’s status quo on abstention, somewhat more modest than what Frankfurter envisioned: a cautious use of abstention in a relatively small number of equitable cases. A second possibility would be a liberal future that backtracks from abstention, as legal liberals recognize a cautionary lesson in Frankfurter’s hostility to an assertive, rights-protecting judiciary. The third future would be one embracing Frankfurter’s vision of abstention in the name of judicial restraint. Abstention has the potential to curb federal court power and, at least on the margins, put more adjudicative power in state courts. This possibility might bring together modern progressives, who are wary about a largely conservative federal judiciary, with conservatives who want to promote judicial restraint and an increase in democratic accountability.