A rise in the power of Islamist parties has lent fresh interest to the old question of how revolutionary movements respond to participation in democracy. Pessimists argue that such movements will use any power they gain at the ballot box to subvert democracy, while optimists believe that participation can turn extremists into moderates. The example of 20th century European communist parties shows that political context can trump radical essence, for where democratic institutions and structures were relatively effective and legitimate, communist parties were moderated by participation. But such evolution was not universal or inevitable, and in weak democracies communist parties remained committed to revolution and presented a real threat to democracy.