opposed to a federalist system of government (= one in which power is divided between a central government and several local ones)

The Foreign Secretary assured anti-federalist MPs that he would not agree to anything that limited British sovereignty. 外交部长向反联邦制度的下院议员保证，他反对任何限制英国君权的行为。

someone who is opposed to a system of government in which power is divided between a single central government and several regional ones

Many voters are staunch anti-federalists. 很多选民是坚定的反联邦主义者。