In the last election, 89 percent of reporters voted for the incumbent. The content of news programs reveals that reporters allowed the personal biases reflected in this voting pattern to affect their news coverage: 54 percent of coverage concerning the challenger was negative, compared with only 30 percent of that concerning the incumbent.
The statement 'ignores the possibility that there was more negative news worthy of reporting concerning the challenger than there was concerning the incumbent' makes the argument logically most vulnerable to criticism.