Murderer + Rapist = Black Man?

In a "news analysis" accompanying the latest New York Times story about the Trent Mouth scandal, Adam Clymer writes:

"There have been occasions when raising the race issue has been both embarrassing and successful [for Republicans]. In the 1988 presidential race, George H.W. Bush was helped by a commercial that attacked Gov. Michael Dukakis of Massachusetts for the furlough of a murderer, Willie Horton, when he raped a woman. His aides insisted that the campaign had nothing to do with the advertisement and seemed chagrined."

From this account, it's hard to understand why criticizing the furlough of a dangerous criminal was considered an example of "raising the race issue." Had Clymer mentioned the color of Horton's skin, it would have been clearer. Or would it?