AP
In his prepared remarks, he warned that the biggest risk to U.S. growth is a "manufactured crisis" like this one.
Social Security payments would be threatened he added.
When the U.S. hits the debt ceiling, it will no longer be able to borrow money by issuing Treasury bonds. This means the government would quickly run out of cash as it pays its obligations.
Some have suggested the government would have to prioritize payments as cash levels fall.
However, Lew warned that there would be "irrevocable damage" from prioritizing payments.