First-term Sen. Robert W. Kasten Jr. (R-Wis.) is having troubles in his reelection bid. Kasten, who won with only 50 percent of the vote in 1980, has been badly damaged by a recent drunk-driving charge in the District of Columbia.

Private polls indicate that Kasten's popularity has dropped significantly since he was charged, leaving Republicans worried. The arrest, coupled with past tax and business controversies, is inspiring a large volume of negative publicitiy in Wisconsin. The Madison Capital Times editorialized that "Bob Kasten is not fit to be a U.S. senator." One of Kasten's potential Democratic challengers, Ed Garvey, former state assistant attorney general and National Football League Players' Association executive director, has been mailing out the press clips.

Garvey and former Democratic Party Chairman Matt Flynn may soon be joined by a third candidate in their primary -- millionaire businessman Herbert H. Kohl. Kohl, who was state chairman from 1975 to 1977, has never held office, but he is not expected to have the usual name recognition problem -- his family once owned 78 department and grocery stores that still bear his name. He also bought the Milwaukee Bucks to keep the professional basketball team in in the state.