BUENOS AIRES — A vote to legalize abortion in Argentina was balanced on a knife’s edge Tuesday as the Senate debated a bill that would make this largely Catholic nation, homeland of Pope Francis, the largest in Latin America to take that step. Legalization in Argentina would reverberate across a region where powerful forces oppose abortion, notably the long-dominant Roman Catholic Church and the fast-growing evangelical Protestant churches. With the bill having cleared the lower house of Argentina’s Congress and the president vowing to sign it, all eyes were on the Senate, where its fat...