Standard & Poor's Ratings Services affirmed its sovereign-credit rating on the United States at its second-highest level, saying the country has a resilient economy but continues to face political and fiscal risks.

S&P affirmed the U.S.'s sovereign credit rating at double-A-plus, and the outlook remains negative.

The ratings firm said the U.S.'s rating reflects the strength of the U.S. economy and monetary system, as well as the U.S. dollar's status as the world's key reserve currency. However, S&P said, the U.S.'s credit weaknesses include the high level of external debt, the recent decline in the effectiveness, stability and predictability of U.S. policy making and political institutions, and recent weak fiscal performance.

Though S&P views the U.S. government and policy making as generally strong, it noted that the ability to implement reforms has weakened in recent years because of a sometimes slow and complex decision-making process, particularly with regard to broad fiscal policy.

The firm said recent shifts in the ideologies of the two major U.S. political parties could raise uncertainties about the government's ability and willingness to sustain public finances consistently over the long term, adding that political polarization has also increased. S&P said it is unlikely that the 2012 presidential elections could resolve the U.S. fiscal debate.

S&P said its base-case fiscal scenario assumes annual real gross domestic product growth of 2% to 3.5% and expects consumer price inflation near 2% through 2016. Without significant fiscal policy change, S&P said it expects U.S. net general government debt, as a share of the economy, to continue to increase after 2016.

The firm said it continues to believe that the U.S. will likely need a more substantial medium-term fiscal consolidation plan than the Budget Control Act of 2011 to stop the deterioration of government indebtedness, as a share of the economy. Such a plan would require broad bipartisan support, S&P said.

S&P sees the U.S.'s risk of returning to recession at about 20%.

In August, S&P removed for the first time the triple-A rating the U.S. has held for 70 years.

Write to Nathalie Tadena at nathalie.tadena@dowjones.com