In an article noting that Karl Rove has been "one of the favorite targets" of Democrats and bloggers, The Hill provides a synopsis of several of the issues that have made Mr. Rove of interest to CREW.  Basically, they all relate to improper activity by one of the highest ranking officials in the Bush administration:  

Prior to recess, Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.) issued a subpoena for Rove to force him to testify before the panel about his role in the firings of several U.S. attorneys. Bush claimed executive privilege to prevent Rove from appearing.

In addition, the House Oversight and Government Reform Committee is investigating whether administration officials are using federal resources for political purposes, a probe that also targets Rove.

As part of both investigations, Democrats are trying to determine whether top White House officials are using non-administration accounts, such as those they received from Bush’s presidential campaign and the Republican National Committee, to evade oversight.

Rove also played a prominent role in the Valerie Plame-CIA leak case and was asked to appear before a grand jury several times. This led Bush’s opponents to hope that Rove, who is one of the most polarizing figures in this administration, would be indicted.