I want to create world where legislative body is full of ideologically pure extremists. I don't want any party that is ready to compromise on its principles. I mean green would be borderline Eco-terrorists, religious would try to outlaw divorce and any religion beside Christianity, socialist would want 75% highest tax brackets and would nationalize every industry they could get their hands on, nationalist would try to expel foreigners etc. The more ideologically pure you are the better. Moderate parties should have trouble to win any seats.
In other words I need some kind of reason why are moderate parties absent from the legislative body.
What kind of political system would reward extremist parties?
First past the post system leads to creation of two very similar parties. So that would probably mean some kind of proportional representation. However even in proportional representation the largest parties are the moderate ones. Extremist become king-makers only if one the moderate parties need their votes badly to pass the budget or some law.
I don't want a system where center right/left parties accept few ideological policies from the extreme right/left in exchange for votes. I want center right/left parties small & weak.