Deep partisan conflict in the mass public threatens the stability of American democracy. We conducted a megastudy (n=32,059) testing 25 interventions designed by academics and practitioners to reduce Americans’ partisan animosity and anti-democratic attitudes. We find nearly every intervention reduced partisan animosity, most strongly by highlighting sympathetic and relatable individuals with different political beliefs. We also identify several interventions that reduced support for undemocratic practices and partisan violence, most strongly by correcting misperceptions of outpartisans’ views – showing that anti-democratic attitudes, although difficult to move, are not intractable. Furthermore, both factor analysis and patterns of intervention effect sizes provide convergent evidence for limited overlap between these sets of outcomes, suggesting that, contrary to popular belief, different strategies are most effective for reducing partisan animosity versus anti-democratic attitudes. Taken together, our findings provide a toolkit of promising strategies for practitioners and shed new theoretical light on challenges facing American democracy.