Political debates over knowledge claims often become emotionally charged, with two sides not only disputing what is true but seeing those on the other side as deluded or worse. By looking at use of the term “Laffer curve” in the U.S. Congress from 1977 to 2010, we draw attention to two ways such debates over knowledge claims can evolve. The Laffer curve is a simple schematic representation of the relationship between tax rates and government revenue that was influential in U.S. tax policy in the late 1970s. Early on, Republicans and Democrats faced off over the Laffer curve as a cognitive symbol to be debated with argument, evidence, and reference to experts. Over time, Republicans continued to treat the Laffer curve as a cognitive symbol, but for Democrats it became a polluted expressive symbol that could be dismissed without debate. Democrats also articulated the Laffer curve as part of an ironic narrative about the failure of the Reagan administration, which ended the possibility of serious deliberation. We suggest that the dynamics seen here may also be present around other politicized knowledge claims, such as the claim that human activity is causing climate change.