CITIZENS UNITED V. FEC…. And the hits just keep on coming.

The Supreme Court has ruled that corporations may spend freely to support or oppose candidates for president and Congress, easing decades-old limits on their participation in federal campaigns.

The court on Thursday overturned a 20-year-old ruling that said corporations can be prohibited from using money from their general treasuries to pay for campaign ads. The decision almost certainly will also allow labor unions to participate more freely in campaigns and threatens similar limits imposed by 24 states.

The justices also struck down part of the landmark McCain-Feingold campaign finance bill that barred union- and corporate-paid issue ads in the closing days of election campaigns.

The full 183-page ruling is online (pdf). It was, of course, a 5-4 ruling. Aren’t they all?

This is not exactly my area of expertise, but at issue is whether corporations and unions can run independent expenditure campaigns for and against candidates. Now, it appears, they can, suggesting that entire system of financing political campaigns in the United States just received a rather dramatic — and more than a little radical — jolt.

Before the ruling, Sen. Russ Feingold (D-Wis.) said the decision had the potential to take the country “not just back to a pre-McCain-Feingold era, but back to the era of the robber barons in the 19th century.”

At first blush, that’s seems to be what the high court has done, clearing the way for unrestricted corporate spending in campaigns. (Remember, if it’s corporations pitted against labor unions, it’s not much of a contest — an ExxonMobil-based independent expenditure campaign in support of a far-right candidate will easily trump anything SEIU or the AFL-CIO might even hope to try.)

For more background on the case, I found this article helpful. I’ll have more on this later.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.