A top political adviser to President Barack Obama blamed the downgrade of the U.S. credit rating on tea party Republicans, whom he said were unwilling to compromise on how to reduce the federal debt.

Obama campaign strategist David Axelrod told CBS' "Face the Nation" on Sunday that the decision by the Standard & Poor's credit agency to downgrade the U.S. from AAA to AA+ for the first time was strongly influenced by weeks of standoff between Democrats and Republicans over the debt.

Axelrod called the action "a tea party downgrade" and says it's clearly on the backs of lawmakers who were willing to see the country default to get their way.

Tea party activists are a loose-knit coalition of community groups made up of people with conserative views; many of the movement's candidates oppose tax increases and back balanced budget amendments.

Axelrod also criticized Republican presidential candidates for not speaking up in favor of compromise.

Earlier, an S&P official said there is a 1 in 3 chance that the U.S. credit rating could be downgraded another notch if conditions erode over the next six to 24 months.

The credit rating agency's managing director, John Chambers, told ABC's "This Week" that if the fiscal position of the U.S. deteriorates further, or if political gridlock tightens even more, a further downgrade is possible.

Chambers also said that it would take "stabilization and eventual decline" of the federal debt as a share of the economy as well as more consensus in Washington for the U.S. to win back a top rating.

S&P downgraded the U.S. rating Friday, from AAA to AA+, for the first time.

Meanwhile, former Federal Reserve Chairman Alan Greenspan said he expects the stock market slide to continue in the wake of a decision by S&P to downgrade the U.S. credit rating.

Appearing Sunday on NBC's "Meet the Press," Greenspan said markets will take time to bottom out and that he expects a negative reaction on Monday to the S&P action.

But Greenspan also said he doesn't see any risk in investing in the United States and says that S&P's downgrade won't change that.

The former Fed chairman said the downgrade "hit a nerve" and is damaging to the psyche of the country. But he said he can't foresee a scenario in which the U.S. will default on its debts.