The final Gallup poll shows that likely voters prefer Republicans to Democrats by a 55% to 40% margin on the congressional ballot. This historical model also predicts that the Republicans could gain anywhere from 60 seats on up. Gains well beyond that are also possible.
Gallup states the following:
It should be noted, however, that this year’s 15-point gap in favor of the Republican candidates among likely voters is unprecedented in Gallup polling and could result in the largest Republican margin in House voting in several generations. This means that seat projections have moved into uncharted territory, in which past relationships between the national two-party vote and the number of seats won may not be maintained.