The U.S. lost its last stable outlook from the three biggest credit-ranking companies after Fitch Ratings lowered the nation to negative following a congressional committee's failure to agree on deficit cuts.

Fitch's outlook on the U.S., which it still assigns its top AAA grade, reflects "declining confidence that timely fiscal measures necessary to place U.S. public finances on a sustainable path will be forthcoming," making the probability of a downgrade greater than 50 percent over two years, the company said yesterday in a statement. Standard & Poor's and Moody's Investors Service said Nov. 21 that the so-called supercommittee's inability to reach an agreement didn't merit downgrades because the inaction will trigger $1.2 trillion in automatic spending cuts.

U.S. government debt rallied the most since the end of 2008 after Standard & Poor's stripped the U.S. of its AAA ranking on Aug. 5, while global equities lost $9.7 trillion in market value during that period. Even with lawmakers reluctant to embrace the automatic cutbacks that helped prevent downgrades, President Barack Obama has pledged to veto any efforts to undermine the spending reductions.