Treasury Secretary Jack Lew said Monday he worries that waiting until the last minute to raise the nation's borrowing authority could result in an accident "that would be terrible."
Last week, Lew said the U.S. debt ceiling will be exhausted Nov. 3, two days before previously estimated. In a letter to congressional leaders, he added that a remaining cash balance of less than $30 billion would swiftly deplete.
"Our best estimate is November 3rd is when we'll exhaust what we call extraordinary measures; those are things we can do to manage things. I will run out of things that I can manage on November 3rd," Lew told CNBC's "Squawk Box."
The federal government is scraping by just under its $18 trillion legal borrowing limit.
Not increasing the debt limit would be "ridiculous," Lew said. He stressed that the risk of not acting is real. "There are people who think we have the ability to choose what day this is. We do our very best to count the numbers ... [and] give Congress the best information we have."
Disarray among Republicans over Rep. John Boehner's successor as House speaker is complicating negotiations. According to an aide, Boehner may try to pass a debt-limit increase before he retires from Congress, which had been scheduled for the end of the month.
Only Congress can raise the debt ceiling. Lew insisted that a hike is not a commitment to new spending but an ability to pay the bills on money already spent. Conservatives have in the past targeted the borrowing limit as leverage in budget negotiations.
Lew dismissed the idea that the government could prioritize what bills to pay. "Once you no longer consider all of your obligations rock solid, you're no longer the full faith and credit of the United States."
"It's also not possible to pick and choose. We have about 80 million transactions a month. Our system wasn't set up not to pay," he added.
A debt-ceiling fight in the summer of 2011 resulted in a credit-rating downgrade of the United States by Standard & Poor's. The Treasury also came close to missing payments in 2013.