Last November, Democrats picked up 6 governorships, held all 13 seats with Democratic incumbents, and retained their one (and only) open seat. But the lesson from Election 2006 is not so much a story of democratic dominance in the fight for the governor's mansion as it is the power of incumbency—Republican incumbents also won 12 of 13 races (92 percent).
Since 1998 there have been 129 gubernatorial elections in the United States. Incumbents have successfully defended 68 of 79 races (86 percent). The success rate of Republican incumbents (41 of 48, 85 percent) is virtually identical to that of Democratic incumbents (27 or 31, 87 percent).
When parties lose control of the governor's mansion, it is usually when there is an open race. In fact, party control has shifted in more than half of the open seat gubernatorial races since 1998 (25 of 48, 52 percent). Democrats (9 of 18, 50 percent) and Republicans 14 of 30, 47 percent) fare about equally well in finding a fellow party member to replace their outgoing state executive.
It is true, of course, that unpopular governors sometimes avoid defeat by not running (e.g. Bob Taft of Ohio in 2006), thus boosting the success rate of incumbents listed above. But that's not always the case—Ernie Fletcher (R-KY) is currently boasting an approval rating in the low 30s, and has launched his re-election bid for this fall's race in Kentucky.