“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.”

In 2006, in Randall v. Sorell, the Supreme Court struck down Vermont’s contribution limits, the lowest in the nation, as unconstitutional. Individuals and political parties were not allowed to contribute more than $400 to a candidate for statewide office over a two-year election cycle, including primaries. In a brief concurrence, Justice Samuel A. Alito Jr. said there was no reason to address the continuing validity of Buckley v. Valeo in that case, suggesting that a later case might present the question directly.

The latest case, McCutcheon v. Federal Election Commission, No. 12-536, may be that case.

The court also issued a pair of Fourth Amendment decisions on Tuesday.

In one of them, the court ruled, 6 to 3, that the police may not stop and detain people without probable cause in connection with a search warrant once they had left the premises being searched.

The case, Bailey v. United States, No. 11-770, concerned Chunon Bailey, a New York man who left an apartment in 2005 as it was about to be searched. The police had a warrant to look for a gun, which they ultimately found. They also followed Mr. Bailey’s car for about a mile before stopping, handcuffing and searching him.

Mr. Bailey was later convicted of gun and drug charges. He asked lower courts to suppress evidence from the stop — statements he made and a key linking him to the apartment — but they refused, relying on Michigan v. Summers, a 1981 Supreme Court decision allowing the detention of people in the immediate vicinity of the place to be searched.

Justice Anthony M. Kennedy, writing for the majority, said none of the interests justifying the detention of people at the scene had allowed Mr. Bailey to be detained. People far from the scene cannot endanger officers conducting the search or disrupt it, he said. Nor could the interest in “preventing flight” be stretched, he wrote, to “justify, for instance, detaining a suspect who is 10 miles away, ready to board a plane.”