If you were right and the main stream media was "left leaning", they would not have feasted on Clinton sexual deviances without comparing these the Newt Gingrich's exact same infractions on our moral code.
If you were right, the main stream media would not have given the swifties exposure ten times greater than the actual advertising buy of the group, that without putting emphasis on the debunking of their claims, nor bringing up the simple fact that if O'Neil claims had an merit, Nixon would have sink Kerry as he did Ellsberg while the history was fresh and traceable. The fact of the matter, a leftist media would have used the swifties add about Kerry testimonies in congress, to point out the courage and determination of Kerry on the battle field first, and then in bringing the Vietnam conflict to an early termination (saving his comrades in arm lives in the process), while risking ruining his future (as Ellsberg actually did).
They surely debunked very rapidly Rather's point on Bush's service, concentrating on the documents rather than the facts of the period. If they were balanced to the left, as you say, they would have used the recent book claiming Bush Jr doing drugs in Camp David, to keep this probably false story (like the swifties story) on the air to "seed doubts" for weeks on, rather than a single mention and rapid death in the news rooms.
If you were right and the media was left leaning, it would have pointed out why drugs from Canada are cheaper, because the government allows the drug companies to shift the R&D burden for world health care on the shoulders of the American public. Nothing in the media on that. Nothing in the media on the fact that middle class retirees getting their dividends from retirements plans pay full taxes on these. The list is too long to continue, I think you are being misled thinking that the mainstream media is left leaning. The media sold its soul for the pound of flesh of massive concentration and consolidation allowed it under current FCC rules and is hopping fo even more. Listen to Red Sumner, "personally I would vote for Kerry, but as the Chairman of Viacom, I have to support Bush, since their policies are better for Viacom."