Sometimes the two U.S. political parties face challenges from insurgents within their own ranks. In 1992, conservative commentator Patrick J. Buchanan took on a sitting president of his own party he felt had betrayed its principles. Vermont governor Howard Dean lead a 2004 uprising that did not prevail on the ballot but altered the political landscape for future Democratic campaigns. From the new PBS series: The Contenders, an eight-part series the reveals the humanity, the twists of fate and the surprising mistakes behind 16 of the most compelling and tumultuous presidential campaigns in modern history. Watch the full series at pbs.org/the-contenders/]]>