For 13 years, Puerto Rico has been in a man-made recession aggravated by natural disasters. In 2017 the island territory asked a federal court for bankruptcy protection, but it’s still grappling with $70 billion in debt. Puerto Rico’s fiscal fiasco offers a glimpse of what the future could hold for Illinois if it doesn’t deal with its own self-imposed economic disaster.

Long the economic hub of the Midwest, Illinois has lost more than 850,000 residents to other states during the past decade. The state has been shrinking for...