Debt held by the public reached $31.27tn by 31 March, topping nominal GDP of $31.22tn for the first time since World War Two.
Gross federal debt has already passed $39tn, while the Congressional Budget Office projects debt held by the public will hit 108% of GDP by 2030 and 120% by 2036.
The Committee for a Responsible Federal Budget blamed bipartisan fiscal inaction and said stabilising the ratio would require about $10tn in deficit reduction; last week's Senate budget resolution offered no structural fix.
With interest payments set to become the largest government program, what happens when America can no longer afford its promises?
Is the 100% debt-to-GDP ratio a true crisis or an outdated metric for the world's dominant economic power?