The poll, released Friday, found Ernst with 45 percent support to Braley with 44 percent. Another 3 percent said they preferred another candidate in the race while 9 percent said they were undecided.
The poll is one of the first surveys of the general election matchup since Braley and Ernst won their party's respective nominations on Tuesday.
The Rasmussen Reports poll was conducted among 750 likely Iowa voters between June 4 and June 5. It had a margin of error of plus or minus 4 percentage points.