Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I say who wrote them.
Squire. 1992. Challenger profile and gubernatorial elections. Western Political Quarterly 45 (March): 125-142.
Jacobson and Kernell (1983) demonstrated the importance of challenger quality ("strategic entry") to Congressional elections. Squire seeks to apply a similar logic to gubernatorial elections. See also Brown (2007).
Squire creates an index of challenger quality rather than just using a dummy as previous studies have done. He codes former governors and senators at 6, U.S. representatives 5, statewide officials 4, state legislators 3, local elected officials 2, other political positions (like party chairs) 1, and those who have held no office in the last 2 years as 0. He then multiples this by a number from 0 to 100, reflecting the percent of the state's population the the individual represented. This has the effect of making U.S. representatives from Connecticut, where there were only 6 House representatives at the time, much more prominent than California representatives.
This index assigns every challenger a score from 0 (no office in the last two years) to 600 (former governors or senators). State legislative leaders are arbitrarily scored at 100.
Only ex-governors and U.S. House members have much luck unseating incumbents (see Table 1), while most other types of challengers succeed rarely. As such, ex-governors and U.S. House members don't appear to behave strategically; they have a good chance at winning whether there is an incumbent running or not, though they do perform better in open contests.
Statewide officeholders are quite strategic. They fare decently in open contests, but poorly when challenging incumbents, so they run more frequently in open contests. This has the effect of preventing lower-quality candidates (state legislators, local leaders, inexperienced, etc) from running in open contests; instead, they serve as sacrificial lambs running against incumbents.
Interestingly, female candidates for governor tend to be of higher quality, though there are very few cases to consider.
Squire uses several variables to predict challenger quality, including term limits, term length, other party's vote share in the previous election, whether the election is held in an odd year, national economic conditions, the size of the high quality challenger pool (i.e. how many opposition party members hold statewide office, House seats, etc), and so on. Of these, only the other party's previous vote share is consistently significant, though the national economy and the size of the high quality challenger pool are significant in some models.
The challenger's party does better depending on the party's strength in the electorate and the current economic conditions.
The index accounts for the correct variables, but Squire presents no evidence that it is valid. Why should we believe that it is? Is there a better way to operationalize challenger quality?
Research on similar subjects