Objectives Washington State held its first “top two primary” in 2008. Under this system, the two candidates receiving the most votes move on to the general election, regardless of party affiliation. This study empirically examines the potential incentive under this top two primary system for each political party to discourage “excess” party candidates from entering primary contests. Methods We examine this possibility by looking at the Washington State Legislative Primaries in 2004, 2006, 2008, and 2010. With these data, we estimate the factors impacting the number of primary candidates in a race for each political party, including the change in the primary format in 2008. Results Our results indicate that the switch to the top two primary reduced the likelihood of having multiple Democratic candidates in a race, reduced the number of “excess” Democratic candidates, but did not have a significant impact on Republican candidates. Conclusion With many states revising their primary systems, an understanding of the incentives present under alternative systems is critical. As such, the results presented in this study provide evidence that the top two primary gives the political parties an incentive to discourage excess primary candidates.