An organization called Media Tenor has begun analyzing the coverage of the presidential candidates, including Bush, on ABC, CBS, and NBC. They have two reports up so far (both PDF).
The first report was released before the Iowa caucuses and covers the period from January 1 to January 15. During this period, Dean received more than five times as much news coverage as any of the other Democratic candidates. In some cases, he received more coverage than Bush. The study notes the emergence of "mean Dean" stories but concludes that they didn't harm his overall positive image. It also looks at what the coverage is focused on and finds (surprise surprise) that it's mostly not the issues. When the issues are discussed, they aren't the ones that voters indicate are most important to them.
The second report was released before the New Hampshire primary and covers the period from January 1 to January 22. It notes that although Kerry received a huge upsurge in coverage after Iowa, he didn't quite catch up to Dean and Dean's coverage decreased only slightly. The new coverage of Kerry was generally positive, but the coverage about Dean became predominately negative. This study also says that positive coverage of Dean began to decrease before the caucuses. The chart shows, however, that negative coverage remained about the same until after Iowa. To me, this suggests that negative coverage did not actually increase, but there was less positive coverage to balance it out so it may have seemed more negative. The first study, as noted, said that there was negative coverage, but it did not significantly harm Dean's positive image. It seems as though the authors of the second study were interpreting the same results in a different way based on what they knew about the Iowa results. Another part of the study looks at coverage of policy-related issues and whether the views of the candidates are provided when these issues are discussed. Bush's stance was cited more than half the time. The views of Kucinich, Lieberman and Gephardt were never cited (Sharpton and Moseley Braun were never mentioned at all; the coverage of Kucinich was minimal). Oddly enough, Clark did the best of the Democratic candidates in getting his stances mentioned, but that was only about 12% of the time and certainly not comparable to Bush. And, coverage of him was much less than that of Dean and Kerry, who got most of the media attention.
It will be very interesting to see the next Media Tenor report.