However, some state, regional and local governments can carry higher ratings than Treasuries, S&P's analysts noted, and so not all will see higher borrowing costs.
"When we know the details of the federal-expense-reduction plans, we will update our ratings" of state, regional and local governments, Steve Murphy, S&P's managing director of public-finance ratings, said on the call with Wall Street analysts and the media.
Standard & Poor's also downgraded the debt of the triple-A rated Federal Home Loan Banks and Federal Farm Credit System as well as the ratings on five triple-A rated U.S. insurance firms that have collateral debt tied to Treasury ratings, and $17 billion in debt issued by five other insurance companies.
S&P lowered the ratings on the insurance firms because the nation's "lower sovereign credit rating constrains the long-term ratings on these U.S. insurers because their businesses and assets are highly concentrated in the U.S. and they have significant holdings in U.S. Treasury and agency securities."
Expect to see many similar downgrades in the weeks ahead.
Some corporate bond ratings could also be affected because the market uses Treasury bonds as a reference when pricing other bonds.
But S&P analyst Laura Feinland Katz said high-grade corporate bonds shouldn't be affected by the government downgrade, especially companies that are globally diversified, are in industries "with low reliance on the public sector, have products for which domestic demand is relatively inelastic and low exposure to regulation."
U.S. banks rated by S&P are unaffected by the government's downgrade, the ratings firm said.
S&P analyst Ronald Barone said budget cuts to trim the deficit could impact everything from Medicare programs to not-for-profit schools dependent on government-backed student loans that cover tuition. So, too, are companies that derive a high share of revenue from government contracts. They range from information-technology companies that provide hardware and support services as well as private prison contractors.
S&P sovereign credit analyst Nikola Swann said his firm's rating of France "remains triple-A and the outlook remains stable" as its government has found a resolution for its deficit problems, including means of raising revenue and implementing pension reforms on a timely basis, something the U.S. has failed to do.
Nevertheless, France had to fend off market rumors Wednesday that it, too, is facing a sovereign-debt downgrade, which was likely sparked by the U.S. cut. Those rumors hurt the euro and stocks of French banks.