Jon Tester, Montana’s buzz-cut farmer-senator, reacted angrily on Monday to the Supreme Court’s summary dismissal of his state’s 100-year-old ban on corporate campaign contributions, calling it another blow to democracy.

“The court’s supposed to be full of smart, well-thought-out people, but they rolled back Montana 100 years, back to the time literally when millionaires and billionaires bought elections, and they did it under the guise of free speech, which is crazy,” he said in an interview. “This is really a sad day in American democracy.”

In a brief unsigned opinion, the court ruled that a law passed when copper barons ruled the state violated the tenets set out by the Citizens United decision, which allowed corporations and unions to contribute to campaign-oriented organizations. Advocates of the Montana law had hoped that a Supreme Court review would give the justices a chance to revisit Citizens United in light of the torrent of campaign money it helped unleash. At the least, Mr. Tester said, the court could have given deference to local laws enacted to address local concerns.

Mr. Tester is locked in a tough re-election fight against Denny Rehberg, the state’s at-large Republican House member. Even before the court’s ruling, Mr. Tester had found himself strafed by Republican groups like Crossroads GPS financed by donors who have largely gone unreported.

“If they are able to paint me as something I am not, they have a chance to win this election,” he said. “And they’re doing a lot to paint me as someone I am not.”

Mr. Rehberg released a statement on Monday that said that free speech, including political speech, is guaranteed by the First Amendment no matter what state you live in.

“For Montanans, this means free speech is protected equally if you are a member of a labor union, a private business or a political party,” the statement said. “Instead of trying to silence political dissent, let’s focus on improving transparency and creating stricter reporting requirements. There’s no excuse for letting powerful special interest groups exert influence on our elections from behind a veil of secrecy and anonymity.”