Do the math: about $250B monthly revenue; about $25B in debt service with plenty of money left over for all truly essential functions like the military.
Socialist Insecurity has its own fund, and by law the checks must go out.
What the politicians may be worried about is a sudden inability of the U.S. Government to continue debt expansion through massive borrowing. Credit demand has collapsed, so no more government debt means the credit bubble may suddenly begin to deflate, triggering a massive deflationary depression.
Of course I believe deflation is inevitable anyway. The question is not if, but when. No debt ceiling increase may provide the tipping point. I think this is what scares the politicians, who apparently believe they can delay the inevitable at least until they are gone from office, in which case their successors will be stuck holding the bag.