Tom Beaumont broke the story on the Des Moines Register blog:

Gov. Chet Culver’s campaign manager said today he has resigned, citing personal reasons for leaving the Democrat’s re-election bid after only three months.

Andrew Roos, who was hired in September to manage Culver’s re-election campaign, said he had decided to step down before Thanksgiving and that Culver did not ask him to quit. According to The Des Moines Register’s Iowa Poll, Culver’s job approval has fallen steadily throughout the year and last month was behind potential Republican challengers in hypothetical match-ups.

Deputy campaign manager Jesse Harris, who was hired along with Roos, will serve as interim manager until a permanent successor is hired, Roos said.

“We’re leaving on totally friendly terms,” Roos told The Des Moines Register. “I want to do whatever I can for the governor.”

Roos declined to elaborate on the reasons for the decision. The 34-year-old native of South Bend, Ind., is single and came to Culver’s campaign after having managed Democrat Jack Markell’s winning campaign for Delaware governor last year.

I hadn’t heard any rumors that this was coming and have no idea what the backstory is. Culver’s campaign aired two television commercials this fall, and I thought the second one was quite good. The latest Iowa poll by the Des Moines register was brutal, but you can’t pin that on the campaign manager.

In related news, Taniel of the Campaign Diaries blog published new governor’s race ratings today. Very few incumbents are secure:

In 2010, 37 states will hold Governor’s races, and at the moment the incumbent party can be said to be safe in only five of them! That is not due to any attempt on my part to pile on races in the “likely retention” category: A full 22 states are here classified in the most competitive categories: 13 are “leans” and 9 are “toss-ups.”

Iowa is rated “lean-takeover.” Of the 10 governor’s seats Campaign Diaries considers most likely to change parties next year, five are currently occupied by Democrats and five by Republicans. Most incumbents have seen their ratings fall because of budget problems and high unemployment rates associated with the current recession.