Lawmakers spent most of Tuesday trying to reach an agreement to lift the government's borrowing limit and avoid an eventual default. The limit is a cap on how much debt the government can accumulate to pay its bills. The government borrows in most years because its spending has long exceeded its revenue.
Fitch is one of the three leading U.S. credit rating agencies, along with Standard & Poor's and Moody's Investors Service.
S&P downgraded U.S. long-term debt to "AA+" in August 2011. But three months ago, it raised its outlook. That was in part because of tax increases and spending cuts that have helped shrink the budget deficit. S&P has said it's unlikely to change its rating because of the debt-limit fight.
Moody's said last week that even if Congress failed to raise the limit by Thursday, Treasury could make its interest payments ahead of other bills, "leaving its creditworthiness intact."
Fitch took a dimmer view Tuesday. It said Treasury might not be able to prioritize its interest payments. "It is unclear whether it even has the legal authority to do so," Fitch said.
A credit rating is an assessment of how able a country or company is to repay the money it's borrowed. A AAA rating lets companies and governments borrow at super-low rates.
So far, most investors have remained confident in U.S. debt, though rates have risen on short-term Treasury bills and shot up Tuesday as Congress' deadline neared. In a rare move, Fidelity Investments and JP Morgan Chase said last week that they had purged their money market funds of all U.S. bills that come due soon after this week.
Still, rates have remained stable on longer-term debt, like the benchmark 10-year Treasury note. That shows that investors remain confident in longer-term Treasurys. The rate on the 10-year note is important because it affects rates on mortgages and many other loans.
Jack Ablin, chief investment officer at BMO Private Bank, said Fitch's warning was no surprise.
"Creditworthiness is a function of two thing: ability and willingness," Ablin said. "No one is questioning our ability. But there is uncertainty about our willingness to pay."
After S&P downgraded long-term U.S. credit two years ago, investors sold stocks but continued to buy longer-term U.S. Treasurys.
Many analysts say further downgrades to the U.S. credit rating would likely have little effect on bond investors. That's because Treasurys are the most transparent and widely traded security in the world. Rating agencies have little information to assess that isn't already available to most investors.
Julian Brigden of Macro Intelligence 2 Partners, an investment consultancy, said Treasury investors may shrug off Fitch's warning, which could get lost amid Washington's budget impasse.
"It's just one more negative in the mix of news," Brigden said.
The U.S. government has never intentionally failed to pay its debts. That's why investors consider Treasurys the safest investments in times of uncertainty. Treasurys are also denominated in dollars, the main currency used by central banks and financial institutions around the world.
Late Tuesday, House Republicans pushed for passage of legislation that would allow the Treasury to borrow normally until early February and end a 15-day partial government shutdown at least until Dec. 15.
While the House readied for a possible Tuesday night vote, the immediate result was to impose a daylong freeze on Senate negotiations on a bipartisan compromise.
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AP Business Writer Bernard Condon in New York contributed to this report.
