Writing for the majority, Chief Justice John Roberts said that the existing contribution limits violated the First Amendment. While noting that some level of government regulation of campaign finance is necessary to prevent and root out corruption, he argued that placing aggregate limits on campaign contributions ultimately stifles constitutionally protected political speech.

"There is no right more basic in our democracy than the right to participate in electing our political leaders ... The government may no more restrict how many candidates or causes a donor may support than it may tell a newspaper how many candidates it may endorse ... We do not doubt the compelling nature of the “collective” interest in preventing corruption in the electoral process. We permit Congress to pursue that interest only so long as it does not unnecessarily infringe an individual’s right to freedom of speech."

How did the dissent respond?

Not cheerfully. In his fervent dissenting opinion, Justice Stephen Breyer argued that the decision directly increases the power and influence of the wealthy elite while muffling the voices of the greater public. Removing spending limits, he said, invites corruption and further encourages a big money, pay-to-play political process.

"In reality, as the history of campaign finance reform shows and as our earlier cases on the subject have recognized, the anticorruption interest that drives Congress to regulate campaign contributions is a far broader, more important interest than the plurality acknowledges. It is an interest in maintaining the integrity of our public governmental institutions. And it is an interest rooted in the Constitution and in the First Amendment itself ... Where enough money calls the tune, the general public will not be heard. Insofar as corruption cuts the link between political thought and political action, a free marketplace of political ideas loses its point."

How did the case originate, and who is this McCutcheon guy?

The case was jointly brought by plaintiffs Shaun McCutcheon, a wealthy conservative Alabama businessman, and the Republican National Committee. McCutcheon had contributed $33,000 to 16 candidates running for federal office in the 2012 election. He said he wanted to give $1,776 each to 12 more candidates, but was prevented from doing so because of the existing spending cap.

In September 2012, a federal district court dismissed the suit on the grounds that the aggregate spending limits adequately withstood First Amendment scrutiny and that the government may justify such regulation as as a means of preventing corruption or the appearance of corruption.

Shortly thereafter, the plaintiffs filed their appeal to the U.S. Supreme Court.

What's the difference between this and Citizens United?

Citizens United v. FEC, the landmark 2010 ruling, struck down limits on independent political spending by corporations and unions. It allowed them to make undisclosed contributions of unlimited amounts of money to “independent expenditure” organizations that work on behalf of candidates but do not directly coordinate with them. A subsequent lower court decision allowed individuals to donate unlimited sums to these super PACs as well. The ruling, however, didn't address caps on direct contributions to candidates.

Conversely, McCutcheon does not apply at all to corporations or unions, and it still requires full donor disclosure. And while the decision gives individual donors the green light to make unlimited aggregate contributions to campaigns or parties, it still limits the amount that can be given to any one campaign or party. But unlike Citizens United, the new flow of money allowed by this decision will be controlled directly by candidates and parties.

McCutcheon v. FECMcCutcheon v. FEC

Explained: The Latest Supreme Court Ruling on Campaign Spending Limits