One more reason to be skeptical of those polls where the sample includes a lot more Democrats than Republicans:
Democrats are significantly less likely now (39%) than they were in the summers of 2004 and 2008 to say they are “more enthusiastic about voting than usual” in the coming presidential election. Republicans are more enthusiastic now than in 2008, and the same as in 2004.
These results are based on a July 19-22 USA Today/Gallup poll. They suggest a shift in Republicans’ and Democrats’ orientation to voting in the coming presidential election compared with the last two, with Republicans expressing more voting enthusiasm. The current 51% to 39% Republican advantage in voter enthusiasm is slightly larger than the 53% to 45% GOP advantage Gallup measured in February of this year.
In the 2008 election, Democratic voters outnumbered Republican voters by a 7 point margin, 39-32 percent; on Election Day 2010 it was evenly split, 36 percent to 36 percent. (Notice that Republicans needed only a tie to run up enormous margins in the midterms.)
While it’s entirely possible that the GOP turnout will be terrific while the Democrat turnout disappoints, I think forecasting a political environment somewhere between the ones of 2008 and 2010 is most likely. A Democratic margin of three to five percentage points is probably fine, and two to six is still plausible. Of course, we’ve seen much larger margins in some recent polls, like the 11 percentage point spread in the most recent NBC News/Wall Street Journal spread – a divide that I think makes the poll pretty useless, unless you genuinely think a lot more Americans will self-identify as Democrats in November than they did in 2008.