For purposes of comparison, take a look at Bloomberg’s March poll. Just three months ago, Bloomberg found Obama and Romney locked in a 47-47 tie. Does anyone seriously believe that Obama’s stumbling campaign has produced a 13-point gain since March? Of course not. The difference lies in the composition of the sample.

In March’s survey, Bloomberg did not ask respondents to indicate whether they were Republicans or Democrats. Bloomberg did, however, ask the generic Congressional preference question, which is a fair proxy. The March respondents (including leaners) favored Republican candidates 46-44. The respondents in yesterday’s poll, on the other hand, favored Democrats 48-41. So there was a nine-point swing in favor of Democrat-leaning respondents, which, combined with the margin of error, accounts for the poll’s skewed result. In June, Bloomberg did ask the party affiliation question; Democrats (again, counting leaners) outnumbered Republicans 38-33. (In the Rasmussen survey, by way of contrast, 33.4% of likely voters say that they are Democrats, compared with 35.7% who describe themselves as Republicans.)