SAN JUAN, Puerto Rico—Puerto Rico has suffered a population slide that is steeper and more financially disastrous than in any U.S. state since the end of World War II.

An exodus of workers, retirees and entire families has shrunk Puerto Rico’s population by more than 9% in the past decade to less than 3.5 million, magnifying the territory’s inability to repay its $70 billion in debt.

Congress...