In 1990, Zimmer ran for the United States House of Representatives for the 12th District, then encompassing parts of Hunterdon, Mercer, Somerset, Morris and Warren counties. The seat was open after six-term incumbent Jim Courter decided to not seek reelection after unsuccessfully running for Governor of New Jersey the previous year. In the Republican primary, Zimmer defeated Assemblyman Rodney Frelinghuysen, the early favorite, and Phil McConkey, former wide receiver for the New York Giants. In the general election he defeated Marguerite Chandler, a businesswoman from Somerset County, by a margin of 66 to 34 percent.