Has the Bush administration politicized corruption investigations?

A very interesting study suggests that the Bush Justice department is investigates allegations of corruption by Democratic officials far more frequently than it does allegations of corruption by Republican officials. Specifically, 78% of the corruption investigations they found had Democratic targets, vs. 18% for Republicans.

This isn't (yet) an open-and-shut case, but it is highly suspicious. Some natural questions:

1. As the authors themselves admit, they didn't find every investigation. They relied on search engines to come up with what they could. But if word of an investigation never reached the press, or if they missed it in their searches, we wouldn't have it.

2. When I look at the site of the people who are publicizing this study (www.epluribusmedia.org), I get the impression that they have an anti-Republican bias. So while it is entirely possible that this study gives the full story, I would not be confident that it does until the whole thing is evaluated by a more neutral source (e.g. Newsweek).

3. In light of points 1 and 2, it would be interesting if the Justice department were to release complete data on public corruption investigations and the party affiliations of their targets.

4. I wonder what the picture was under past administrations? For instance, do Democratic administrations tend to disproportionately investigate Republicans?