This seventeenth chapter explores political realignments and third-party movements as they have been specifically harnessed by progressive populism associated with the political left, and nationalistic populism associated with the political Right. Populism, though frequently associated with the right, is not ideologically valanced. First, progressive populism was both a social and political movement broadly calling for reformation, rectification, and restitution. From inception to end, we can see the banner of American progressivism begin as a social movement in the hands of Jane Addams and Jacob Riis, continue as a political movement influencing Republicans under Theodore Roosevelt and Democrats under Woodrow Wilson, and then briefly become strong enough to support Robert La Follette’s presidential bid as a third-party progressive candidate before the movement’s force was finally absorbed by FDR’s New Deal, which moderated America’s ethic of self-reliance (Howe, 2009) with a slate of socialistic programs. Whether as a social movement advocating reform, a political force operating on extant parties to secure voting rights, or as an independent party deploring child labor, multilevel selection allows us to see the unity in the progressive movement’s advancement of individually selected rights. Second, nationalistic populism began as sentiment among the masses and variously manifested as a movement and as a party. As reviewed in the prior chapter, the nativist Know-Nothing American Party challenged the Whigs from without as a third-party movement, bringing about a realignment to homogeneity, nationalism, traditionalism, and other mainstays of the conservative temperament. Also considered are war, immigration, geopolitical strength, and related conditions (Wimmer, Waves of war: Nationalism, state formation, and ethnic exclusion in the modern world. Cambridge University Press, 2013) that elicit nationalistic sentiments and elect nationalistic leaders. Both the progressive and nationalist movements have suffered the same fate as other third-party movements: Their ideological force was absorbed by an existing party, necessitating some movement toward one or the other ideological pole; either to individually or group selected ideologies latent in the body politic.