After the Swedish parliamentary election of 2018, it took 134 days to install a new government. This is both the longest government formation process in the country’s history, and also long by comparative standards. What caused the delay? The most immediate culprit was of course the rise of the Sweden Democrats (SD), but SD entered Swedish parliament already in 2010, without causing any evident delays in the government formation process either then or in 2014. Something else must thus have happened in 2018 that changed the other parties’ stance toward SD, in particular in relation to government formation. The purpose of this paper is to explain, through a mixed-methods design, the prolonged bargaining duration in the Swedish case. We combine a large-n study of 356 government formation processes in 17 West European parliamentary democracies in 1945-2018 with an in-depth case study based on 37 interviews with leading Swedish politicians. Taking the election results as a given, we follow the literature in combining preference uncertainty and bargaining complexity among the political parties as the two main explanatory mechanisms at work. We hypothesize that the prolonged bargaining duration was mainly the result of, first, the strong pre-electoral commitments made within the center-right “Alliance for Sweden” that now had to be undone, with complicated intra-party decision making processes within two of the parties (the Center Party and the Liberals) as a result; and second, the  climate of distrust between these two parties and the incumbent government, in particular the Social Democrats, causing bargaining failure and hence delay. This time, the Swedish parliamentary democracy stood the test; but if prolonged bargaining duration becomes a recurrent pattern in the future, it might be under threat.