The Treasury markets taxpayer debt securities throught its primary dealers, shown at this link:
http://www.newyorkfed.org/markets/pridealers_current.html
The heads of those banks are the go-to people that are needed to keep the bond sales moving.
When Hank Paulson (ex-CEO of Goldman Sachs) told GWB that the "banking system would collapse", he meant that a number of the major banks would be insolvent, ergo, this primary dealer banking syndicate would cease to function, i.e., Congress can't borrow money.
Hank's solution was a boatload of government borrowing and to use the cash to extend gracious loans to banks as necessary to ensure they have enough cash to meet their current obligations, without having to start selling off assets to raise cash, which would not have worked for every bank all at once, of course, since the problem already was rooted in declining values of financial assets.
The reliance of the US government on this banking syndicate is such that Hank could easily ask for and get clearance to start the taxpayer commitment at $700 billion by means of the quickly-passed
Emergency Economic Stabilization Act of 2008.