A new Mason-Dixon survey, conducted for the St. Louis Post-Dispatch and News 4, finds presumptive GOP nominee Mitt Romney leading President Obama 51-42 percent among registered voters.

Obama holds a 51 percent unfavorable rating in the state with 34 favorable, challenging numbers three months from election day. Romney holds a 39 percent favorable rating to 27 unfavorable.

In the Senate race, the poll finds incumbent McCaskill trailing all three GOP primary contenders. McCaskill would lose a head-to-head matchup by nine points to businessman John Brunner (R), 52-41 percent, by eight points to former state Treasurer Sarah Steelman (R), 49-41 percent, and by five points to Rep. Todd Akin (R), 49-44 percent.