Will Republicans gain the net 40 seats they need for a majority in the House? Several factors will certainly help.
One of the asymmetries of American politics is the Republican advantage in the House of Representatives. Evidence: in 2004, George W. Bush, winning 51% of the national popular vote, carried 255 of the 435 congressional districts, while John Kerry carried only 180. In 2008, Barack Obama, winning 53% of the national popular vote, carried 242 of the 435 congressional districts, while John McCain carried 193.
To try to get an idea of how many districts Republican and Democratic candidates would carry if they got the same percentage of the national vote, I used this handy website that shows the percentage of vote for the Republican and Democratic candidates in each of the 435 House districts in 2000, 2004, and 2008.
First, I estimated the number of districts Bush would have carried in 2004 if he would have won 53% of the vote, by assuming that his percentage rose 2% in every district and Kerry’s percentage fell by 2%. Result: Bush would have carried 15 more districts than he actually did. Second, I estimated the number of districts Obama would have carried in 2008 if he had won 51% of the vote, by assuming that his percentage fell 2% in every district and McCain’s percentage rose by 2%. Result: he would have lost 18 districts that in fact he won.