Many items in the Federal Budget are "non-discretionary" meaning that Treasury authority to spend for these items is not a voted appropriation every year. Many are probably familiar with the major entitlement programs such as Social Security and Medicare, the appropriations for these two major programs that account for a large percentage of Federal spending are actually not voted on by the Congress, they are part of non-discretionary spending. The amounts that recipients receive is based on past amounts, annual COLAs are computed by BLS and Treasury automatically adjusts the deposits made to the enrolled citizens.
Another item that is not voted on is the interest the Federal government pays on the "debt", that is interest on Treasury securities. From Wikipedia: "Funds to make federal interest payments have been automatically appropriated since 1847". So Congress has not voted on paying the interest on the "debt" in 163 years! The Treasury Dept. has enjoyed 163 years of autonomy in arranging these payments.
I wonder if the debt doomsday crowd realizes that for the US to default on it's interest obligations, the Congress and the President would have to take a positive action to pass completely new legislation that would overturn a law that has 163 years of legal precedent?
This does not seem likely to say the least.