In Citizens United, the Supreme Court ruled that the only justification for limiting campaign expenditures was "corruption or the appearance of corruption." And since independent expenditures, including those from corporations and unions, don't have any kind of corrupting influence, there's no justification for limiting them.
Many of you will have two responses to this:
Both of these are legitimate reactions. The first because the court's handwaving on corruption got a lot less attention than all the fuss over Citizens United granting corporations free speech rights on a par with individuals. And the second because the court didn't really bother justifying its belief that independent expenditures don't corrupt the political process. It just said there wasn't enough evidence of corruption to survive strict scrutiny and left it at that. However, Rick Hasen notes that a recent case involving a Montana law that limits campaign expenditures prompted a statement from Justice Ruth Bader Ginsburg suggesting that she wants to use the case to expose this for the specious reasoning that it is:
The Montana court  upheld a state ban on corporate campaign spending, finding that Montana's history of corruption justified the ban…Everyone expects the Supreme Court to reverse the Montana case, likely on a 5-4 vote. But the big question is how the court will do so.
…Justice Ginsburg agreed that staying the Montana ruling was the right course, because lower courts are bound to apply Supreme Court precedent even if it is wrong; it is for the Supreme Court to fix its own wrong precedents. But then she added these words in a statement for herself and Justice Stephen Breyer with respect to the stay: "Montana's experience, and experience elsewhere since this Court's decision in Citizens United…make it exceedingly difficult to maintain that independent expenditures by corporations 'do not give rise to corruption or the appearance of corruption.' A petition [to hear the case] will give the Court an opportunity to consider whether, in light of the huge sums currently deployed to buy candidates’ allegiance, Citizens United should continue to hold sway."
…The statement in Citizens United that independent spending cannot corrupt or undermine the public's confidence in the electoral process is a fiction which defies common sense. In fact, the greatest danger of super PACs is not that they will influence the outcome of elections, but that contributions to these groups will skew public policy away from the public interest and toward the interest of the new fat cats of campaign finance. The public, too, seems greatly concerned about money this election season.
It's probably unlikely that the Montana case will lead to a reversal of Citizens United. But at least it will force the court to tackle the least defensible part of its reasoning head on.