"The debate has lasted longer and been more intractable than we had expected," said John Chambers, a managing director at S&P, in an interview after U.S. markets had closed.
U.S. government debt has long been viewed as the safest investment in the world. A credit downgrade could push interest rates higher and send stock prices lower. Treasury officials have warned it could trigger another global financial crisis.
So far, though, financial markets have shown little sign of concern about the government's creditworthiness. Yields on the benchmark 10-year note remain low by historic standards at around 3%. Investors continue to pile into short term Treasury bills as they seek safety from the European debt crisis, pushing yields to near zero.
Mr. Chambers's comments conflict with the beliefs of some lawmakers that Congress could wait until after Aug. 2 to raise the debt cap and avoid having the government's debt downgraded.
S&P has assigned a top-notch AAA bond rating to U.S. debt since 1941, but on Thursday took the unprecedented move of putting short- and long-term U.S. debt on a status termed "CreditWatch negative." The rating agency said there was now a 50% chance it could downgrade U.S. debt within three months.
S&P said it could downgrade U.S. debt even if the debt ceiling is raised "if we conclude that future adjustments to the debt ceiling are likely to be the subject of political maneuvering to the extent that questions persist about Congress's and the Administration's willingness and ability to timely honor the U.S.'s scheduled debt obligations."
And it warned that failure to reach an agreement to reduce the budget deficit by $4 trillion over 10 years also could lead to a downgrade because it could show an "inability to timely agree and credibly implement medium-term fiscal consolidation policy."
The criticism was the latest in a series of warnings from rating agencies about both the risk of not raising the debt ceiling and not reaching a significant deficit-reduction plan.
The White House has said the warnings show the urgent need for action to raise the borrowing limit, while some Republicans have said they show the need for swift deficit reduction.
On Wednesday, Moody's Investors Service placed its Aaa bond rating of U.S. government debt on "review for possible downgrade" and said there was a "rising possibility that the statutory debt limit will not be raised on a timely basis, leading to a default on U.S. Treasury debt and obligations."
Moody's also warned of future downgrades to the debt of Fannie Mae and Freddie Mac, as well as debt issued by the governments of Israel and Egypt that is backed by the U.S. Moody's said the probability of a U.S. default on its debt was "low" but no longer "de minimis."