@Timocracy The US borrows in its own currency which it can print, so if they default it's because they decided to protect the value of the dollar at all costs. So hold USD.
This is an extremely weird scenario though, conventionally a country that owes its own currency would devalue rather than default.
@Timocracy So if the thought is, "I have this exact scenario in mind involving default", maybe describe it and we can think how to handle it.
If it's "stuff is weird and the national debt seems like a lot and maybe everything will go to shit", the answer is a mix of stocks and bonds, foreign and domestic. Also add some productive crypto assets with a plausible story for how they create value and return it to tokenholders (most don't) as a defence against institutions failing.
@Timocracy talk to a financial adviser if you have unusual circumstances but literally doing nothing is quite likely the best option for the vast majority of people.
Retirement savings are by definition very long term and default, while possible, is definitely short term. It’s not like accounts will go to 0, expected cash flows on impacted bonds will accrue and get paid later.