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.
Turett. 1971. The vulnerability of American governors, 1900-1969. Midwest Journal of Political Science 15 (Feb): 108-132.
Commentators have suggested the governors have become more vulnerable--that their incumbency presents less of an advantage. Turett counters this view, showing that American governors have become more 'visible,' but they are no more 'vulnerable.' Also, urbanization, crime, taxes, spending, and unemployment do not correlate with gubernatorial vulnerability. In fact, Turett found no pattern to vulnerability at all; it appears completely random.
Presidential contests can influence gubernatorial races, but only when the presidential contest is a landslide. Otherwise, coattails appear minimal. This is true even in states that continue to hold gubernatorial elections concurrently with presidential ones.
Research on similar subjects