The Supreme Court delivered a political shocker today, overturning a law that limited the amount of money corporations and labor unions can spend on candidates.

The move is widely considered a boost for Republicans in the upcoming midyear elections.

The court's decision struck down parts of a 2002 campaign finance law crafted by U.S. Sens. John McCain and Russ Feingold. The 5-4 decision was a bitter one, reports The New York Times, and "was a doctrinal earthquake but also a political and practical one."

Before now, companies and unions weren't allowed to spend money on television ads or billboards supporting (or booing) a federal candidate, according to the Los Angeles Times. But the court said the First Amendment does allow for that.

Experts tell the Times that the decision opens the floodgates for millions of dollars to pour into political advertising this fall. Republicans, they said, will benefit the most.

Companies still can't contribute directly to a candidate, however, the Times reports.

One dissenting justice, John Paul Stevens, described the decision as "a radical change in the law," one that "dramatically enhances the role of corporations and unions -- and the narrow interests they represent -- in determining who will hold public office."