Though he admits that it wouldn't be successful, Ambinder thinks it could prove annoying:
It conveys maximum anger and carries with it an implicit threat. Even if the left can't find someone who could beat Obama, they could make his reelection campaign much more challenging, and certainly create the type of distractions that a sitting president does not need. For liberals, this might not lead to disaster, especially if it looks like Republicans will nominate a candidate like former Alaska Gov. Sarah Palin, whom they believe is easily beatable.