jats:pThis article examines the origins and influence of ideological index scores—where liberal and conservative interest groups rate legislator performance on selected roll call votes. Two such groups founded in the mid-twentieth century—the Americans for Democratic Action (ADA) and the Americans for Constitutional Action (ACA)—were crucial to the development of this type of metric, transforming roll call analysis from detailed tabular scorecards into streamlined percentage scores showing how often a lawmaker voted “right.” ADA and ACA scores have been heavily utilized in political science as proxies for liberalism and conservatism and used to demonstrate the growing polarization of the congressional parties. Archival evidence suggests, however, that those scores were intended to create the very phenomenon they have been used to measure. They were deeply political rather than objective metrics, which the ADA and ACA used to guide their electoral activities in accordance with an increasingly partisan strategic plan. Each group directed campaign resources toward incumbent lawmakers they rated highly, but they did so unevenly—with the ADA favoring liberal Democrats over Republicans and the ACA showing a preference for conservative Republicans over time. By rewarding favored lawmakers in their preferred party, and using scores to highlight and discourage ideological outliers, they hoped to reshape the parties along more distinct and divided ideological lines—to create more “responsible” parties, as prominent political scientists then desired.</jats:p>