Elections are defining moments in which the stability of democracies is tested. The behavior of losing parties is key in this stage of the democratic process. In authoritarian regimes, losing parties might reasonably challenge the outcome of elections as a consequence of widespread and systematic fraud that altered the outcome of the election. However, post-election disputes have also occurred in 21% of the democratic presidential elections worldwide since the beginning of the third wave of democracy. Why so? I argue that in presidential democracies, losing political parties are not rejecting the outcome of the election to publicize fraud but rather to induce the winning party to negotiate benefits for the losing party. Using an original dataset that codes the behavior of runner-up candidates in 180 presidential elections (1974-2012) and 966 years of electoral legislation, I find evidence that, in democracies, losing parties with an unfavorable negotiating position in Congress are more prone to dispute presidential election outcomes, even after controlling for the quality of the election and the margin of victory. The cases of the presidential elections in Venezuela in 1978 and Indonesia 1979 illustrate how the distribution of seats in congress is connected with election disputes. I test the same argument at the sub-national level in Mexico. If the theory holds at the national level, it should also survive a test at the sub-national level. The sub-national analysis includes a quantitative comparison of the 74 gubernatorial elections that have taken place since the transition to democracy and semi-structured interviews I conducted with 21 runner-up candidates from most of the states in Mexico. The interviews help to identify the politicians' motivations for challenging election results, and to illustrate how post-electoral negotiations happen.