WASHINGTON (Reuters) - The U.S. Supreme Court on Friday handed a victory to corporations and a political advocacy group by allowing them to spend freely before Montana's 2012 elections, a follow-up case to the court's major campaign finance ruling two years ago.

The justices granted a request from the three plaintiffs to put on hold a December decision by the Montana Supreme Court that upheld a century-old state law banning independent corporate campaign spending.

Lawyers for the plaintiffs said the Montana decision contradicted the U.S. Supreme Court's ruling two years ago that gave corporations the constitutional free-speech right to spend freely to support or oppose candidates for federal office.

The Montana Supreme Court ruled the U.S. Supreme Court's decision two years ago did not control the outcome because the Montana law was different and justified by the state's interest in preventing corporate corruption and influence in politics.

The Supreme Court's order cleared the way for corporations, unions and other groups to spend unlimited amounts of money on ads and other political activities designed to influence voters.

Montana's primary elections are on June 5 and general elections are in November.

"While I'm disappointed that for the first time in 100 years Montanans won't be able to rely on our corporate spending ban to safeguard the integrity of our elections, I am encouraged that the Supreme Court will give this careful consideration," Montana Attorney General Steve Bullock said in a statement.

"For more than a century, anyone has been able to participate in Montana elections - even out-of-state corporate executives. All we required is that they used their own money, not that of their stockholders, and they disclosed who they are," Bullock added.

Justice Ruth Bader Ginsburg, joined by Justice Stephen Breyer, cited the experience in Montana and elsewhere since the ruling two years ago and said it was difficult to maintain that corporate spending does not give rise to corruption or the appearance of corruption.

She said an eventual appeal in the Montana case will give the court an opportunity to consider whether the ruling two years ago should continue to hold sway "in light of the huge sums currently deployed to buy candidates' allegiance."

The next step in the case will be for plaintiffs to file their appeal with the Supreme Court seeking to overturn the state court ruling.

There has been a massive increase in corporate spending during the political campaigns ahead of the 2012 elections, mainly due to the ruling in 2010.

The Supreme Court case is American Tradition Partnership v. Attorney General of Montana, No. 11-A762.

(Additional reporting by Laura Zuckerman; Editing by Stacey Joyce)

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