What can corporations do in elections now that they could not do before Citizens United ?
Citizens United made only one change in the law by permitting corporations to make independent expenditures to support or oppose candidates. All the other laws regulating corporate (and union) participation in elections, such as being able to make campaign contributions through their PACs, remain the same. As a practical matter, it is tax-exempt nonprofit corporations, not for-profit businesses, that have taken full advantage of Citizens United.
Defenders and opponents of the decision differ over its significance. Defenders focus on its practical effect and point to the comparative trickle of business corporation money that has found its way into elections to argue that the decision was not as important as its critics claim. Opponents say that the real significance of the decision lies not in its immediate practical effect but in the political theory behind it. By declaring corporations to be members of our democratic community, with much the same political rights as citizens, the court took it upon itself to redefine our democracy by fiat. Very few of us are political philosophers, but the overwhelming popular opposition to Citizens United suggests that most of us knew at once that the court had acted against widely and deeply held opinions about what it means to be a democracy and how our democracy should work.