Treasury Secretary Timothy Geithner has made yet another attempt to convince conservative lawmakers they have no choice but to raise the debt limit.

In a lengthy letter sent to 17 Republican senators Wednesday, Geithner offered a broad takedown of the argument that the debt limit need not be raised to avoid a default. He called such an idea "radical and deeply irresponsible," "unwise," "untested and unacceptably risky" and "futile."