The move follows a rating downgrade of the sovereign by another ratings agency, Fitch, this year, which came after months of political brinkmanship around the U.S. debt ceiling.

Federal spending and political polarization have been a rising concern for investors, contributing to a selloff that took U.S. government bond prices to their lowest levels in 16 years.

"It is hard to disagree with the rationale, with no reasonable expectation for fiscal consolidation any time soon," said Christopher Hodge, chief economist for the U.S. at Natixis. "Deficits will remain large ... and as interest costs take up a larger share of the budget, the debt burden will continue to grow."