be partisan

See: adhere
References in classic literature ?
It does not become us, perhaps,' pursued the Dean, 'to be partisans.
Now, some people say to me, 'Oh, you're being partisan and we can't be partisan and you can't be partisan in the pulpit.
It is time to recognise that High Court judges can be partisan and that in the war between the Government and the BBC, sensible people will be backing the BBC to avoid the consequences of politicians seeking to neutralise this vital institution.
He says that being partisan for the sake of being partisan is small-minded--but that being partisan when the facts lead you to be partisan is the only thing to be.