The article shows how the Polish Senate’s electoral rivalry has changed after the electoral reform in 2011, when a plurality rule in single-seat constituencies was introduced. Based on the analysis of the election results, it was shown how individual parliamentary parties adjusted to the new realities. In particular, it was checked to what extent these changes led to phenomena commonly recognized in political science – polarization, duopolization and disproportionality as well as division into safe and marginal (swing) districts. The analysis focused on the strategies of political parties. It turns out that the reform resulted in a few expected phenomena. At the same time, there were clear deviations from the predictions, resulting from the fact that the Senate elections were of a secondary importance. One of the reasons behind the deviations was additional uncertainty in the Senate’s rivalry introduced by the fuzzy configurations of political alliances, especially regarding the nominations.