Jeffrey Toobin on the Citizens United decision:
How Chief Justice John Roberts orchestrated the Citizens United decision, New Yorker: ...In one sense, the story of the Citizens United case goes back more than a hundred years. It begins in the Gilded Age, when the Supreme Court barred most attempts by the government to ameliorate the harsh effects of market forces. In that era, the Court said, for the first time, that corporations, like people, have constitutional rights. The Progressive Era, which followed, saw the development of activist government and the first major efforts to limit the impact of money in politics. Since then, the sides in the continuing battle have remained more or less the same: progressives (or liberals) vs. conservatives, Democrats vs. Republicans, regulators vs. libertarians. One side has favored government rules to limit the influence of the moneyed in political campaigns; the other has supported a freer market, allowing individuals and corporations to contribute as they see fit. Citizens United marked another round in this contest.
In a different way, though, Citizens United is a distinctive product of the ... aggressive conservative judicial activism of the Roberts Court. It was once liberals who were associated with using the courts to overturn the work of the democratically elected branches of government, but the current Court has matched contempt for Congress with a disdain for many of the Court’s own precedents. ... Roberts, more than anyone, shaped what the Court did. As American politics assumes its new form in the post-Citizens United era, the credit or the blame goes mostly to him.
Even if you take a market based approach to the problem, there's still a reason to limit the influence of particular groups of individuals, i.e. those with the money to put into politics. A "free" market monopolized by a few individuals is not optimal, and that's true in political markets as well. The need for more institutional structure of the type Democrats favor to make these markets more competitive -- i.e. to remove the ability of those with money to have an undue influence on the outcome -- seems clear.