Interest paid annually on the US National Debt already exceeds the defence budget. But things are about to get worse. The US tends to borrow short term (5 years or less) and a wall of covid era debt is about to mature and will need to be rolled over.
The problem is the yields on existing debt are about 1.8%. The Biden administration was hoping that yields would have fallen by now to about 3.7% (still nearly double what they're paying). But instead bond yields are spiking back up to about 4.25%
So why are yields spiking? The bond markets think both presidential candidates will cause inflation to rise. Harris by continuing Biden's runaway spending, and Trump with tax cuts that are not offset by spending cuts.
As a result anticipation of further interest rate cuts either in December or in early 2025 are being unwound.
Speculators are scenting blood - people like Stanley Drunkenmiller have openly said they're shorting long duration Treasury bonds, and likely many others are too. They'll keep pushing till the government goes into a panic and makes serious attempts to reduce the deficit by a mixture of spending cuts and tax rises. The panic will likely happen after the election.
As far as I can tell, the only solution is for Americans to elect a fiscally conservative Congress who will tie the President's hands, and that means splitting the ticket - say Harris as President and the Republicans holding both houses of Congress. If one party controls the presidency and Congress, all bets are off.