More fun with numbers

The three latest generic polls of the battle for control of the House all point strongly in favor of the Republicans, with two of them suggesting that the Republican gains could be of historic proportion.
A CNN poll has the Republicans holding a 10 point generic lead. A Fox News poll has the Republicans up by 13 points. Finally a Washington Post/ABC News poll puts the Republican lead at 4 points. These polls, viewed collectively, were taken during the period October 25-30. CNN’s, the most recent, was taken between the 27th and the 30th but had the smallest sample size (542 likely voters).
There is a tendency, I believe, for the generic polling to understate Republican performance. Not all of those who identify themselves as “likely voters” end up voting, of course, and in many years Republicans tend to do their best with those who are most likely to vote. This tendency may be more pronounced this year (the enthusiasm gap).
But let’s put this consideration aside. The average of the three latest polls is Republicans plus 9. If Republican candidates for the House were to outpoll Democratic candidates by this margin on Tuesday, I believe this would translate into a pick-up of 60 to 70 seats. I say this because in 2004, Republicans won the “popular vote” in House races nationwide by 7 percentage points. In the process, they ended up with 232 seats. To match that number of seats, the Republicans would need to pick up 55 seats. A 9 point margin would, other things being equal, mean more pick-ups than that.
The 13 point lead Fox News is showing translates, by similar reasoning, into a Republican gain of 80 seats or more. Even the smaller 4 point lead the Washington Post/ABC News poll reflects would probably mean a gain of around 50 seats.
So at a macro-level at least, 50 to 80 seats looks like the range and a pick-up of 60 seems probable.