This paper investigates the causal effects of the main demonstration organised by a wider social movement in Spain (15M) on short and medium-term electoral results. Using a unique self-constructed database of the number of demonstrators in the main demonstration of the 15M movement, I estimate the electoral effects of the movement six months, four, five, six and eights years after the movement took place. Using unpleasant weather (either rainy or too hot) as an instrument for the number of participants in the demonstration of the 19th of June 2011, I find that in regions that hosted relatively bigger demonstrations, voting patterns corresponded more closely to the messages spread by the movement compared to other regions and that those effects last over time, with dynamic differences depending on the political offer in different elections. Surprisingly, results also show a negative effects on the vote for the far-right party Vox in elections 8 years later. I also investigate the mechanisms and find evidence supporting an increased concern about corruption as well as the creation of a network on social media around the 15M movement that was used afterwards by the new party Podemos to spread its message. Taken together, these results show that new social movements can have medium term lasting effects and can notably change the composition of a country’s Parliament.