By PETER LANDERS

WASHINGTON—The U.S. Supreme Court declined to reconsider its 2010 Citizens United ruling lifting restrictions on corporate and union political contributions, summarily overturning the Montana Supreme Court in a case involving a Montana state law limiting corporate political spending.
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In an unsigned decision, the court said "there can be no serious doubt" that the holding in Citizens United v. FEC applies to the Montana state law. "Montana's arguments in support" of the state court ruling "either were already rejected in Citizens United, or fail to meaningfully distinguish that case," the decision said.
Four liberal justices dissented from the decision. Justice Stephen Breyer, writing for the dissenters, said he would have preferred to reconsider Citizens United or at least its application to the Montana case. He said Montana's experience "casts grave doubt on the Court's supposition that independent expenditures" by corporations and unions "do not corrupt or appear to do so."
The majority turned away pleas from the court's liberal justices to give a full hearing to the case because massive campaign spending since the January 2010 ruling has called into question some of its underpinnings.
The same five justices said in 2010 that corporations have a constitutional right to be heard in election campaigns. The decision paved the way for unlimited spending by corporations and labor unions in elections for Congress and the president, as long as the dollars are independent of the campaigns they are intended to help. The decision, grounded in the freedom of speech, appeared to apply equally to state contests.
But Montana aggressively defended its 1912 law against a challenge from corporations seeking to be free of spending limits, and the state Supreme Court sided with the state. The state court said a history of corruption showed the need for the limits, even as Justice Anthony Kennedy declared in his Citizens United opinion that independent expenditures by corporations "do not give rise to corruption or the appearance of corruption."
Twenty-two states and the District of Columbia, as well as Sen. John McCain and other congressional champions of stricter regulations on campaign money, joined with Montana.
Two liberal justices who were in dissent in Citizens United—Ruth Bader Ginsburg and Stephen Breyer—already had challenged Justice Kennedy's view that the independent campaign spending could not be corrupting by virtue of the absence of links to a campaign.
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When the court blocked the Montana ruling in February, Justice Ginsburg issued a brief statement for herself and Justice Breyer saying that campaign spending since the decision makes "it exceedingly difficult to maintain that independent expenditures by corporations 'do not give rise to corruption or the appearance of corruption.'"
Justice Ginsburg appeared to be referring to the rise of unregulated super PACs that have injected millions of dollars into the presidential and other campaigns. She said the case "will give the court an opportunity to consider whether, in light of the huge sums currently deployed to buy candidates' allegiance, Citizens United should continue to hold sway."
The corporations that sued over the law said it could not remain on the books after the Citizens United decision.
Montana urged the high court to reject the appeal, or hold arguments and not issue what the court calls a summary reversal. The prevailing side in the lower court almost always strives to avoid high court review. But Montana and its supporters hoped a thorough debate over the Citizens United decision would lead to its reconsideration or at least limits on its reach.
—The Associated Press contributed to this article.