Zimmer entered the race for the Republican nomination for the U.S. Senate from New Jersey on April 11, 2008, after being drafted by New Jersey Republican leaders. Party leaders had originally supported businesswoman Anne Evans Estabrook for the Senate nomination until she withdrew in March 2008, following a mini-stroke. Many Estabrook supporters then supported businessman Andy Unanue for the Senate nomination. Unanue received criticism in the race because of his residency in New York City and his spending his entire three-week campaign in Vail, Colorado. Several days after filing his petitions for the Senate race, Unanue dropped out of the race and his committee on vacancies designated Zimmer to enter the race under the Unanue petitions.