Research on the rise of populism has largely overlooked the explanation populists themselves advance: that they fill political representation gaps, defined as discrepancies between mainstream parties’ policies and the “popular will.” We test this claim in an information-provision experiment conducted in the weeks leading up to the 2025 German federal election. A sample of 5,040 German citizens was randomly assigned varying information about the immigration stance of Germany’s mainstream center-right CDU—an issue marked by a substantial representation gap. We find that perceptions of the CDU’s position significantly affect both vote intentions and incentivized behavioral measures: when the CDU is perceived as closer to the electorate’s conservative preferences on immigration, support for the right-wing populist AfD declines. Our estimates indicate that the AfD’s vote share would shrink by as much as 75% if the CDU adopted its immigration stance. These results suggest that the electoral success of populist parties is strongly linked to genuine policy preferences, rather than being driven solely by dissatisfaction with political elites or protest voting.