The perception exists that the flagrant bias pro-Obama and againt Hillary started when this became a 2 candidate race.

Early in the primary campaign, it was apparent that “A” list blogs did not care for Hillary.  But the coverage was relatively fair until Edwards dropped out making it effectively a two-person race.  Some of the “A” listers were overtly supporting Obama, the rest leaned toward Obama while claiming neutrality.  The difference in these positions could be described as “sins of commission” versus “sins of omission.”

These studies were made as of last year on Mainstream media

It should be noted that Air America and the blogosphere went quite closely on the same bias throughout the campaign: ABC (anyone but Hillary). Once Edwards was out, things just became more stark, easier to see – and the persecution of Hillary and her supporters lost any restraint.

here are the old studies:

Radio coverage October 2007 – note the favorable coverage for Hillary:

,Project for Excellence in Journalism study found these numbers last October
Obama: 47% positive, 16% negative.
Clinton: 27% positive, 38% negative.
McCain: 12% positive, 48% negative
Giuliani: 28% positive, 37% negative
JEDNE

Net numbers

Obama +31
Giuliani -9
Clinton -11
McCain -36

and this is a cartoon from January: