From what I can see, most football fans (well, most people in general) in the north of England are left-wing. Of course, it is hard to be a leftie while supporting a professional football club in this age of football as a capitalist institution but a lot of fans have managed it from a variety of clubs. I am loathe to say Liverpool is any more left-wing than any other British club as anecdotally, while I have known Liverpool supporters who are left-wing, I have also known BNP types to support the club (to pre-empt some rather obvbious criticism, this is likely to be the case with every other club as well). And as admirable as the boycott of The Sun is, it relates to a specific incident in which the paper published lies relating to Hillsborough, making false accusations against the fans, so I do not consider it to bear any relation to left-right politics.
As others have said, St Pauli is the most left-wing club in Europe and incredibly, this goes for those who run the club as much as the fans. Those who have been to their home games have told me how impressive the whole club is and I greatly admire the whole set-up of their club. Some internet research would suggest there are plenty of other clubs, such as Omonia Nicosia, who have an admirably left-wing fan base too but I doubt those who run most clubs respect this as much as St Pauli.