STEPPING DOWN. ROBERT MICHEL, 70, U.S. Representative from Illinois and House Republican leader. The Peoria Congressman, who last week announced his decision not to run in 1994, was first elected to the House in 1956, and became minority leader in 1981. He has been known for conducting legislative business in a spirit of traditional collegiality with his Democratic opponents, and this style has increasingly appeared to be at odds with the approach of younger and more combative conservatives like the G.O.P. whip, Georgia's Newt Gingrich--Michel's most likely successor in next year's leadership race.