Although I don't doubt that most of the left-leaning media sincerely believe that they fairly cover Republicans and Democrats, in practice they believe that (D) stands for "defensible" while (R) stands for "reprehensible."
And their coverage follows that distinction. Senate Majority Leader Harry Reid is a case in point (tip to Instapundit):
Another man might have assumed, correctly, that launching a campaign of insult and insinuation against two billionaires would result in renewed attention to his own finances. Not Harry Reid. The Senate Democratic leader since 2005, and the Senate majority leader since 2007, is not one to reflect before speaking. His mouth runs far ahead of his brain.
The man is Exhibit A in the "imagine the press coverage if he was a Republican" game.
But Reid has no reason to be worried. No matter how far ahead of his brain that his mouth runs, the press corps will be behind both, covering up the mess. Reid's wealth, accumulated despite being a life-long "public servant," is defensible, after all.
UPDATE: Thanks to Stones Cry Out for the link. Although in my defense I was speaking of how the media interprets "D" and "Rs."