2)  We cannot predict which races will have a Republican or Democratic skew.

But even if we somehow could say “on average, the polls will be skewed toward Republicans or Democrats” in a given year, to be useful, we’d have to have reason to suspect that the skew applies more or less evenly across races.  But this doesn’t appear to be the case.

For example, Blumenthal — who, again, doesn’t endorse this approach — presents data from 2010 showing a skew of about 3.1 points toward Republicans in the close races. If you move the data 3.1 points toward Democrats, you’d have avoided missed calls in Nevada and Colorado, but you’d have gotten Illinois wrong. In 2006, you would have predicted sizable losses for George Allen and Conrad Burns by correcting for the overall skew of the polls. Yet those two races went against the grain and were razor close, and they were almost the difference makers in Senate control that year.