Question: Will mainstream political parties not cooperate towards outsider parties if they are too extream?
Sentence: Furthermore, in the case of far-right, far-left and regionalism parties in the national parliaments of much of the European Union, mainstream political parties may form an informal cordon sanitarian which applies a policy of non-cooperation towards those "Outsider Parties" present in the legislature which are viewed as 'anti-system' or otherwise unacceptable for government.
Answer in sentence: yes