Interest on U.S. debt is becoming a top driver of future deficits, as the sheer size of past borrowing overwhelms the fiscal outlook 

U.S. debt is expected to continue soaring in the coming decades not because of excesses committed by future lawmakers, although that’s certainly possible, but because interest payments on past borrowing will increasingly dominate spending.

Americans got a reminder of that outlook as the U.S. crossed a grim fiscal threshold this week, when first-quarter data confirmed debt is now bigger than the economy.

As of March 31, debt held by the public stood at $31.27 trillion, while...