WASHINGTON — In a brief unsigned decision, the Supreme Court on Monday declined to have another look at its blockbuster 2010 campaign finance decision, Citizens United v. Federal Election Commission.

In the 5-to-4 ruling on Monday, the court summarily reversed a decision of the Montana Supreme Court that had upheld a state law limiting independent political spending by corporations. That decision, the United States Supreme Court said, was flatly at odds with Citizens United, which said the First Amendment allows corporations and unions to spend as much as they like to support or oppose political candidates.

“The question presented in this case is whether the holding of Citizens United applies to the Montana state law,” the opinion said. “There can be no serious doubt that it does.” Montana’s arguments, the opinion continued, “either were already rejected in Citizens United, or fail to meaningfully distinguish that case.”

The four members of the court’s liberal wing dissented in an opinion by Justice Stephen G. Breyer, who said that Citizens United itself had been a mistake.