"Getting there was difficult, but we shouldn't miss the fact that they did take an action that was moving in the right direction," Hess said of the government compromise last week. "We're not just looking at the politics. There's lots of other things that go on here."
To keep its Aaa ranking, the U.S. needs to take steps to rein in the debt-to-gross domestic product ratio to "not far above" 75 percent by about 2015, which is the ratio that's forecast for 2012, Moody's said in the report. A rating cut may be triggered before 2013 by weakening "fiscal discipline" or a worsening economic outlook that causes "adverse fiscal implications."
"Another factor that we are going to be watching is the performance of the economy," Hess said. Negative outlook means there's some possibility that the U.S. rating could go down anytime during the next two years, he said.
On Aug. 3 Moody's said the outlook for the U.S. grade is now negative, after President Barack Obama signed into law a plan to lift the nation's borrowing limit and cut spending following months of wrangling between Democratic leaders and Republican lawmakers.