Chapter 6 explores co-optation, or the argument that whenever third parties become a threat to a major party, that major party undermines the third-party’s agenda by stealing its issues and rhetoric. The chapter demonstrates that co-optation is a good short-term explanation of third-party decline. It can explain why parties like the Populists in the 1890s and Progressives in the 1910s rapidly collapsed after rising quickly, for example. However, it does not explain the long-term disappearance of third parties in the middle of the twentieth century, since there were no third-party challenges for the major parties to respond to in the first place. The chapter also demonstrates that the New Deal cannot explain the demise of left third parties, since these parties had largely disappeared by the time FDR entered office.