Actually it's a myth that we have always rigidly been a 2 party system. However, we mostly have been; currently have 2 political parties: the Evil Party and the Stupid Party.

Whatever the virtues (and motives) of Manchin's position, it seems to me that he negotiated in bad faith. He had the power at the moment, but that was due to the circumstances, which won't last. The mistrust in himself that he's generated will last forever.