Some of Washington’s less reliable partners are worried too: Top diplomats from Russia and China joined a rare meeting of world powers’ envoys on Capitol Hill this week with roughly 30 Senate Democrats to tamp down concerns over the nuclear agreement.

“The prospect of the rejection of a deal makes us nervous,” Philipp Ackermann, the acting German ambassador to the United States, said Thursday. “It would be a nightmare for every European country if this is rejected.”

The closed-door meeting, which was held Tuesday, sought to dispel criticisms and answer questions ahead of an expected September vote by Congress. That vote will determine whether to continue current U.S. sanctions against Iran and blow up the nuclear agreement, or let the sanctions expire over time as promised in exchange for Tehran rolling back its nuclear program.