“Even a short-lived default could threaten economic growth. It creates the risk of roiling markets, and of sapping economic confidence, and it would prevent Americans from receiving vital services,” the six former secretaries said in the letter https://home.treasury.gov/system/files/136/Former-Treasury-Secretaries-Letter-to-Congressional-Leadership-on-the-Debt-Limit-9-22-21.pdf, released by Treasury. “It would be very damaging to undermine trust in the full faith and credit of the United States, and this damage would be hard to repair.”
The letter was not signed by living former Republican Treasury secretaries Steven Mnuchin, John Snow, nor James Baker. Those signing were Democrats Jacob Lew, Timothy Geithner, Lawrence Summers, Robert Rubin and Michael Blumenthal. Republican former secretary Henry Paulson also signed.