Rating agency Moody's Investors Service on Friday downgraded its outlook on the U.S. government to "negative" from "stable," citing the rising risks to the nation's large fiscal deficits and declining debt affordability.

"Continued political polarization within U.S. Congress raises the risk that successive governments will not be able to reach consensus on a fiscal plan to slow the decline in debt affordability," Moody's said in a statement, as Congress faces the looming threat of a government shutdown once again.

Moody's has affirmed the long-term issuer and senior unsecured ratings of the United States at Aaa. "In the context of higher interest rates, without effective fiscal policy measures to reduce government spending or increase revenues, Moody's expected that the U.S. fiscal deficits will remain very large, significantly weakening debt affordability," it said.

Moody's is the last of the three major rating agencies to maintain a triple-A rating for the U.S. government.

Following months of political brinkmanship over the U.S. debt ceiling, Fitch cut the U.S. long-term foreign currency issuer default rating to AA+ from AAA in August, joining S&P which has had an AA+ rating since 2011.

Moody's warned of "downside risks" because of a rising budget deficit with no apparent plan to rein in the deficit at a time of significantly higher interest rates from the Federal Reserve.

The agency cited a string of recent red flags, including brinkmanship over the debt limit, the ouster of the House speaker and rising threats of another government shutdown.

The Republican-controlled House of Representatives is now under pressure to take a stopgap spending measure aimed at averting a partial government shutdown.