In the 2003 case McConnell v. Federal Election Commission, the Supreme Court upheld key provisions of the McCain-Feingold campaign finance reform law. The FEC writes that the decision upheld restrictions on donations to political parties and limitations on election advertising by outside groups.
The FEC notes that the court struck down other provisions in the law, including a ban on contributions from minors and restrictions on how parties and candidates were allowed to coordinate their election activities.
Seven years later, the court overturned some of its previous rulings in McConnell v. FEC as part of the Citizens United case. The court ruled that outside groups could spend money on advertisements that expressly urged viewers to vote for or against a given candidate, according to the Center for Responsive Politics. The decision led to the rise of so-called "super PACs," which allow outside groups, including corporations, unions and non-profit groups to raise and spend unlimited amounts of money on elections, so long as they do not directly coordinate with candidates and their campaigns.