In Citizens United, the Supreme Court put together two bad lines of precedent to form something even worse. In one line, spending money was held to be a form of free speech, based on some private remark Thomas Jefferson once made. How is speech free, if you have to pay for it? In another line, corporations were held to be "persons" who themselves had all the rights in the Constitution. This was a misinterpretation of earlier precedent that corporations and other fictional entities (like unions) were the means by which natural humans expressed their speech. Put these two together, and you have Citizens United, which basically says it is a violation of the Constitution for the government to regulate spending money on politics by corporations - basically legalized bribery.