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.
Tompkins. 1989. Have gubernatorial elections become more distinctive contests?. Journal of Politics 50 (Feb): 192-205.
With time, gubernatorial elections have become more isolated from national trends--that is, the presidential coattails are weaker. The movement in many states toward holding gubernatorial elections in "off" years has apparently contributed to this change. Further evidence that the gubernatorial and presidential elections have become distinct from one another: Republicans have had increasing success in presidential elections, but gubernatorial elections remain a Democratic stronghold.
Primary conclusion: "In a variety of respect, the electorate voting for governor must be viewed as responding differently from the electorate voting for president" (p 204).
Tompkins pays insufficient attention to the studies that were coming out in the years prior to his own. Peltzman, Kenney, and Chubb had already laid much of the groundwork that current (as of 2006) studies are using to study gubernatorial elections, yet Tompkins does not cite them. This undermines his criticism of the literature: "It is apparent that we are slow in gathering evidence which would allow us to understand these differences better and to assess their implications" (p 204). Still, this critique remains accurate; the academic community has still paid little attention to these problems.
Research on similar subjects