There's a school of thought that people tend to vote for the presidential candidate that they like the most -- the candidate they would personally like to spend some time with (i.e., the candidate they would invite to their home for dinner, or the one they would like to have a beer with). While I personally believe most people vote issues, party, or their fears, I will accept that at least some vote on likability.
So, I bring you the results of this Rasmussen Poll -- done on June 12th and 13th of a random national sample of 1,000 likely voters, with a 3 point margin of error. Make of it what you will.