Ideally they should be neutral. They seem to swing towards whatever viewpoint is going to be the most controversial at the moment though, whatever is going to sell the most papers, rather than have a definite left/right wing bias.
Despite what right wingers say there isn't any "liberal media bias", and if anything I think they're softer on republicans than democrats. They'll go as overboard as they can while still appearing to have some integrity with stories about either party, but with republicans they keep it polite and business like, and tend to gloss over or neutrally portray any wrong-doing. With democrats however they go whole hog and don't spare any lurid details. The republicans would probably argue that it's because democrats are naturally more perverted and lack "good old-fashioned moral values", but we all know that each party has about an equal share of depraved or corrupt human beings. My theory is that republicans=bigger money, therefore republicans get off easier. After all, they wouldn't want to offend any potential investors, business partners, or advertisers, would they?