The purpose of this study is to examine the strategic choice of mainstream parties in relation to the competition of voters posed by a niche party and their most important issue, in this case radicalist rightwing populists and the migration issue. The study uses a comparative approach to examine the mainstream parties Social Democrats and Moderates reaction to the niche parties New Democracy 1991-1994 and Sweden Democrats 2010-2015. Using Meguid´s PSO-theory and by performing an qualitative analyse of the parties rhetoric and political suggestions in the parliamentary debates as well as in government bills and reservations in committee reports, the study aims to describe mainstream parties position on the issue and if and how they change position and strategy. The results of the study shows that both mainstream parties over all applies an adversarial strategy, aiming to maintain distance to the niche party and its position but with time and due to changes in the political environment, changes in position and strategy takes place and the mainstream parties applies a slightly more accommodative strategy.