Why do we see such a strong relationship? It’s not precisely that the number of candidates causes a party to win more seats. After all, there are only so many House seats in play. What a large number of challengers does create is a better recruitment environment. If there are several challengers from whom to choose in a particular race, a party can pick the strongest nominee.
Political science research suggests that the recruitment of high-quality candidates explains a good deal of election outcomes — if a party can convince a large number of skilled and experienced candidates to run for office, those candidates tend to do better and the party tends to win more seats. Indeed, the recruitment of quality candidates helps explain the development of the incumbency advantage in 20th century American politics. Finding strong candidates was Newt Gingrich’s approach prior to the 1994 Republican landslide, just as it was Rahm Emanuel’s strategy for 2006.