According to Charlie Cook (per DKos), Ohio’s Second Congressional District, which the Republicans just held by a 52-48 vote, was about 13 points more Republican than the country as a whole in the last two Presidential elections.
What would happen if that were the pattern nationally: i.e., if the Republican Congressional candidate in each district got a vote share that was 12 points below Cook’s Partisan Voting Index for that district? How many seats would change hands?
Update The consensus seems to be that a swing that big would move 70 seats into the Democratic column, but that the more modest but still substantial swing indicated by current national polls wouldn’t even give the Democrats control of the House, due to the thoroughness with which the Red State Leninists have carried out their gerrymandering.