Recent events across Europe suggest that parties and voters may be more willing to tolerate cooperation with far-right and anti-democratic parties at the European level than they are at the national level. In Germany, for example, all mainstream parties have sworn off cooperation with the far-right AfD, at least in part because they believe that their voters would not support it. However, the German center-right, and indeed many center-right parties across Europe, regularly cooperate with parties that hold far-right views, such as Fidesz in Hungary, in the European Parliament and other EU institutions. Using an original survey experiment with over 8,000 respondents, this project explores the extent to which cooperation among parties at the national and EU levels is tolerated by voters and why. The paper addresses questions including the following: Does cooperation with extreme parties at the national level actually lead voters to view the party they support more negatively? Do voters overlook cooperation with extremists at the EU level simply because they are unaware of it, but punish it as they would at the national level once made aware of it? Or do voters view cooperation at the supranational level less harshly than they do at the national level? Do they react similarly to parties on the far-left as they do to parties on the far-right, and to parties taking authoritarian stances, regardless of ideological positioning?