Michigan's campaign finance laws initially prevented corporations from contributing to political campaigns or purchasing political advertising out of the company's general fund. The Chamber of Commerce felt the law - the Michigan Campaign Finance Act -- was an unconstitutional violation of their free speech and sued. In a 6-3 decision, the Supreme Court declared that the law was narrowly constructed and served a compelling government interest: the reduction of corruption induced by corporations funding politicians favorable to their interests. Therefore, it didn't violate the Constitution. This ruling -- Austin v. Michigan State Chamber of Commerce -- opened the door for potent state and federal campaign finance reform laws.
In 2010, the Supreme Court heard the case of Citizens United v. Federal Election Commission and overturned the Austin decision. The 5-4 decision essentially drew two conclusions: Money equals speech, and corporations have the same right to free speech as individuals. Therefore, corporate political spending can't be illegal. The concurring opinions (opinions agreeing with the ruling, but adding additional details) were careful to consider the role of stare decisis and why it was important to overturn the ruling despite legal precedent. The dissenting opinion was pointed in observing the ruling's "rejection of the common sense of the American people … few outside the majority of this Court would have thought [American democracy's] flaws included a dearth of corporate money in politics."
The Citizens United decision also overturned parts McConnell v. Federal Election Commission, a 2003 Supreme Court decision that upheld the McCain-Feingold Act, which introduced federal campaign finance reforms.