A backup plan to cut the federal deficit and keep the U.S. government from default gained momentum Thursday even as President Barack Obama and congressional leaders paused their negotiations to determine if they can reach a deal.

Ratcheting higher the pressure on Washington to strike a deal, Standard & Poor's for the first time said there was a 50% chance it would downgrade its rating of long-term U.S. debt within three months because the chances of default were "increasing" and the political debate about deficit reduction...