One commercial accuses President Obama of worsening the deficit and says, “It’s time to take away Obama’s blank check.” Another attacks Republican tax and Medicare policies, arguing: “We can’t rebuild America if they tear down the middle class.” So begins the shadow campaign of 2012, in which a new breed of “super PACs” and other independent groups is poised to spend more money than ever to sway federal elections. The first major ads look as if they came from a regular campaign.