WASHINGTON (AP) — The weekend deal spelling out how Iran will roll back its nuclear program in exchange for sanctions relief buttresses the Obama administration’s argument Tehran may be prepared for a grand diplomatic compromise averting the potential for war. But it has done little to sway skeptical lawmakers determined to levy new sanctions against Iran.

With world powers and Iran set to start the clock on their six-month interim agreement Jan. 20, a parallel showdown looms between President Barack Obama and Congress over legislative action each side says has serious implications for the chances of diplomatic success. Obama warns adding more sanctions could kill negotiations; legislators insist they’re the only way to ensure Iran keeps its word. Much could depend on Tehran quickly making good on its commitments.