On March 1, 2010, Halter announced his candidacy in the 2010 U.S. Senate election, challenging two-term incumbent Blanche Lincoln in the Democratic primary. After one month of fund-raising, the Halter campaign announced that it had raised more than $2 million, mostly from out-of-state and from national labor unions. On May 18, 2010, Halter and a minor third candidate prevented Lincoln from polling over 50% of the vote, thus triggering a runoff on June 8 between Lincoln and Halter, which Lincoln won. Lincoln ultimately lost the general election to her Republican opponent, United States representative John Boozman.