The political left can connect her to her husband, and social conservatism, and connect that to the Tea Party.
And that damages the Tea Party image and effectiveness, compared and contrasted to the movement's original premise.
That premise was to stay well clear of the social issues, and focus on fiscal issues.
But with Bachmann as the head of the Tea Party caucus, it is impossible to follow that premise, considering her husband says he can convert homosexuals into heterosexuals.
This makes the job of the left too easy, and costs the movement votes they might otherwise gain.
So it is too bad they let her be the head in the House. Now she is running for President again, or threatening to run, like others that have effectively taken up entertainment as their new field (Palin, Gingrich, Buchanan, Keyes, Santorum, Huckabee, etc.)