Argentina's presidential election battle lines have hardened after economy minister Sergio Massa entered the race in a dramatic late twist to take on front runners including a conservative city mayor, ex-security czar and libertarian economist. The most notable late confirmation has been Economy Minister Sergio Massa, whose candidacy was announced somewhat unexpectedly Friday night. "This completely changes the political scene," said Alejandro Corbacho, director of political science program at Argentine University UCEMA.