We use novel data to describe the evolution of party differences in parliamentary speech in Finland during 1907–2018. We find a peak in left-right polarization in the 1970s, driven by the extreme left party, and co-occuring with a high prevalance of Soviet Union related phrases, perhaps resulting from Soviet information influencing. The period was also marked with short-lived coalition governments and inefficient policymaking. Moreover, as we find that left-right partisanship fluctuates during the majority of the 20th century, our results show that the levels of polarization currently perceived as high in many countries may not be that exceptional.