The flip side is that many bond funds, insurance companies and retirement plans have requirements that they only invest in AAA rated bonds. If treasury bonds are downgraded, then these will all be dumped on the market immediately... or more likely the boards of directors of those funds will quietly amend their operating rules from "AAA" to "AAA or U.S. government bonds".
But if they do have to dump them, then bonds will be in oversupply and rates will go up.
I wonder if the raters would cause that much trouble in the market without reason. “Deadlock” wouldn’t give them reason to downgrade us. Of course after a bad debt-ceiling rise, which is what we’ll probably get, is passed then they would have reason.