We conduct a laboratory experiment to study the incentives of a privileged group (the “yellows”) to share political power with another group (the “blues”). The yellows collectively choose the voting rule for a general election: a simple-majority rule that favors them, or a proportional rule. In two treatments the blues can use a costly punishment option. We find that the yellows share power voluntarily only to a small extent, but they are more inclined to do so under the threat of punishment, despite the fact that punishments are not subgame perfect. The blue group conditions punishments both on the voting rule and the electoral outcome.