Wisconsin Democrats and their allies who are trying to take out Republican Gov. Scott Walker have invested all their hopes in Mary Burke, a Harvard-educated political newcomer whose father started Trek Bicycle when she was a teenager. For Democrats and their friends in organized labor, this race is personal. They mean to avenge Walker's evisceration of union power as he builds his resume for a possible presidential run. But for Burke, the campaign also poses an awkward challenge: She can't talk too stridently about her opponent's most provocative actions for fear of alienating independent voters, many of whom supported both Walker's union crackdown and President Barack Obama's re-election bid. And they will decide this contest, too.