The Treasury is walking a tightrope on U.S. debt by relying so much on short-term rates that are at the mercy of a suddenly very hawkish Fed

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The federal government has been running on a hamster wheel of debt by refinancing trillions of dollars every month with trillions more in fresh borrowing that comes due in a few months.

To keep interest costs on $39 trillion in debt from exploding further, the Treasury Department has relied heavily on short-term securities that have lower yields than longer-term bonds.