The Supreme Court has reaffirmed its decision to allow corporations to spend unlimited funds on federal and state elections and struck down a 1912 Montana law that prohibited such expenditures.

The 5-4 decision extinguished a glimmer of hope for critics of the 2010 Citizens United ruling who had hoped the high court would use the Montana case to re-examine that decision. But their argument, that the landmark campaign finance decision has corrupted the US political process, was flatly denied by the Supreme Court’s five conservative justices.

The one-page decision by the majority said that the high court’s decision in Citizens United, which allowed both companies and trade unions to spend freely on political advertisements, applied to Montana and therefore made a state law banning such expenditures unconstitutional. Critics of the earlier law had hoped that the Montana case would at least generate a new round of legal arguments before the high court, but the justices made their decision without a rehearing of the issues.

The court has maintained that corporation and unions share the first amendment right to free speech that individuals do under the US Constitution.

The ruling was celebrated by Republican supporters of the Citizens United decision.

Mitch McConnell, the top Republican in the Senate, said that publicly available records showed “minimal corporate involvement” in the 2012 election cycle and that not a single Fortune 100 company was contributing to so-called super-Pacs that are raising tens of millions of dollars to support candidates.

“Clearly, the much predicted corporate tsunami that critics of Citizens United warned about simply did not occur,” he said.

But a minority of high court justices disagreed with that view.

“Montana’s experience, like considerable experience elsewhere since the court’s decision in Citizens United, casts grave doubt on the court’s supposition that independent expenditures do not corrupt or appear to do so,” said Justice Stephen Breyer.

Although publicly traded companies have been slow to take advantage of the decision, Citizens United has indirectly contributed to the rise of super-Pacs and non-profit groups that are being funded by wealthy individuals and are spending hundreds of millions of dollars on presidential and congressional races.

“Clearly the Supreme Court has decided to wash its hands of the disastrous results of its earlier decision,” said Paul Ryan, an attorney at the Campaign Legal Center, which supports campaign finance laws. “Apparently the same five justices who gave us Citizens United are not troubled by the fact that special interests are picking the winners and losers in our federal and state elections.”

Democrats in Congress are pressing for legislation that would increase disclosure requirements for political organisations which currently keep their donor bases secret.

Chuck Schumer, the senior Democratic senator who is leading the charge for such legislation, said of the Montana ruling: “The Supreme Court is further tipping the balance of power in America in favour of deep-pocketed, outside interests.”