Repurchase agreements are short-term trading in government securities. The global repo market is in the trillions. Default would cause the price and value of US government securities to wobble badly, and that would cause sympathetic detonations in financial markets across the world.
Investopedia. Definition of ‘Repurchase Agreement – Repo’
A form of short-term borrowing for dealers in government securities. The dealer sells the government securities to investors, usually on an overnight basis, and buys them back the following day.
Repowatch estimates the global repo market at $7 trillion.