Howard Zinn argues that all history is partisan, it is just that some historians make a pretence that they are presenting straight history rather than their own prejudices (Interview, Socialist Review, June 2007).
I agree that historians need to take sides, but something else needs to be added. The historical method lies in testing your theories, ideas and biases against historical fact, as checked in original records and archival material.
That material may lead you to challenge your earlier assumptions and perhaps even change them. Of course the whole thing is a partial process-the historian chooses which areas to research and what areas to look at, but there is still a reality test.
Whatever our views, there are such things as demonstrable historical facts. We may for example think that Tony Blair has been an awful prime minister while there may still be the odd person (Dave Cameron perhaps?) who thinks he was OK. We can all agree however that he was in fact prime minister from 2 May 1997 to 27 June 2007.