Romney's campaign did not respond to requests for comment, but part of his reasoning could be because Walker's star power burns brighter in the state than Romney's. Walker received a whopping 627,000 votes in his uncontested primary last month, nearly double the 347,000 votes Romney received when he won Wisconsin's Republican primary.

Some Democrats and union officials have criticized Obama for keeping his distance from the race, while Republicans have had no complaints about Romney's lack of involvement. The two aren't directly comparable, though: Obama could have had more impact by helping turn out minority voters in Milwaukee with a campaign visit, while it's unclear how much Romney could have helped the race.