We evaluate how traditional parties may respond to populist parties onissues that are particularly fitting for populist messages. The testing groundis the 2020 Italian referendum on the reduction of members of Parliament (MPs).We implement a large-scale field experiment, with almost one millionimpressions of programmatic advertising in 300 small municipalities. Ourtreatments are video ads against voting “Yes” to the MPs reduction, aconstitutional reform pushed forward by the populist Five Stars Movement. Weadminister both an “informative” video on the likely costs of cutting MPs—aimedat deconstructing the populist narrative—and a “reducing-trust” video—aimed atattacking the credibility of populist politicians. Our field experiment showsthat the latter video is more effective at capturing the viewers' attention.Both videos decrease voters’ turnout and, to a lesser extent, the “Yes” votes.These results show that programmatic advertising is a cost-effective tool todemobilize voters. And that it is easier to convince voters leaning towardpopulist parties to abstain, rather than winning them back to vote fortraditional parties. In other words, demobilization works better thanpersuasion, although this strategy may have detrimental effects on politicalparticipation and social cohesion.