SHE'S the Venezuelan government's most detested adversary, a young woman with a quick wit and machine-gun-fast delivery who often appears in Washington or Madrid to denounce what she calls the erosion of democracy under President Hugo Chávez. In a highly polarized country, María Corina Machado has emerged as perhaps the most divisive figure after Mr. Chávez, a woman who is either beloved or reviled. Ms. Machado, 38, attractive and a fluent English speaker, is lionized by her allies in the opposition as a worldly sophisticate fighting for democracy. But she is demonized by the government, which characterizes her as a...