The chart above is from the Pew Research Center. It is from a survey they did between May 29th and August 25th of this year. The survey questioned 1,550 adults, and has a 2.1 point margin of error.
It shows that Democrats currently have a 9 point advantage over Republicans (51% to 42%). The remaining 7% refused to lean toward either party.
The chart below shows where each party has its strength.