The Supreme Court on Monday turned away a plea to revisit its 2-year-old campaign finance decision in the Citizens United case and instead struck down a Montana law limiting corporate campaign spending.

The same five conservative justices in the Citizens United majority that freed corporations and labor unions to spend unlimited amounts in federal elections joined Monday to reverse a Montana court ruling upholding the state's century-old law. The four liberal justices dissented.

"The question presented in this case is whether the holding of Citizens United applies to the Montana state law. There can be no serious doubt that it does," the court said in an unsigned opinion.

The Citizens United 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."