A narrowly divided Supreme Court on Monday reaffirmed its landmark 2010 decision allowing corporations to spend unlimited money on elections, deciding that a state court was wrong to uphold Montana’s century-old ban on political spending by businesses.

The curt 5 to 4 decision — two paragraphs issued without hearings or debate — further inflamed a national argument over the role of big money in politics, which has become a central feature of the expensive race for the White House between President Obama and Mitt Romney.

A constellation of independent groups is poised to spend $1 billion or more on the 2012 elections, much of it raised in secret from billionaires and corporations. The spending is made possible in part by the court’s 2010 decision in Citizens United v. Federal Election Commission , which found that companies and unions have a free-speech right to donate unlimited amounts for and against candidates.

This atmosphere of fevered spending had triggered hopes among critics that the court might reconsider Citizens United. But Monday’s decision appears to scuttle any chance of that, at least for now.

Obama administration spokesman Eric Schultz said the White House was “disappointed.”

Citizens United mistakenly overruled long-standing cases that protected the fairness and integrity of elections,” Schultz said. “Unfortunately, the court today missed an opportunity to correct that mistake.”

The case involved a Montana law forbidding corporate political spending. The law dated to 1912, when “copper kings” and other mining barons largely controlled the state’s politics. The Montana Supreme Court said that, even after Citizens United, the legacy of corruption and other factors unique to Montana justified a ban on spending by corporations regulated by the state.

But the same five justices who formed the majority in Citizens United said in an unsigned opinion that Montana’s arguments “either were already rejected in Citizens United, or fail to meaningfully distinguish that case.” The decision had the effect of overturning Montana’s law.

Justice Stephen G. Breyer penned a short dissent for the minority, writing that “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.”

Justice Elena Kagan — who argued the Citizens United case as Obama’s solicitor general — joined Breyer, Ruth Bader Ginsburg and Sonia Sotomayor in the dissent. Although Breyer and Ginsburg said in February that the court should use the case to revisit Citizens United, Breyer wrote Monday that he did “not see a significant possibility of reconsideration” by the majority.

Monday’s decision drew strong condemnations from activists who favor tougher limits on money in politics and effusive praise from those opposed to such regulations.

“Citizens and the nation are not going to accept the Supreme Court-imposed campaign finance system that allows our government to be auctioned off to billionaires, millionaires, corporate funders and other special interests,” said Democracy 21’s Fred Wertheimer, a longtime activist who helped draft many of the nation’s post-Watergate election laws.

James Bopp Jr., an attorney for the Montana plaintiffs and a key architect of national challenges to campaign finance laws, called the decision “excellent” and said it “shut the door” on reconsideration of Citizens United.

“This effort has failed and the Citizens United decision is now settled law,” Bopp said in a statement. “Groups of persons of average means will still be able to pool their resources to effectively participate in our democratic process.”

Richard Hasen, a law professor at the University of California at Irvine, wrote on his election law blog that the outcome of the Montana case is actually a “relative victory for campaign-finance reformers” because the five-justice majority shows no signs of budging on Citizens United. “Taking the case would have made things so much worse,” Hasen wrote.

The five justices who made up the majority in Citizens United have consistently held that many legislative attempts to control the influence of money in politics run afoul of constitutional guarantees of free speech. Justice Anthony M. Kennedy, who wrote the Citizens United decision and is often the deciding vote when the court splits ideologically, is a leader on that issue.

Besides lifting the ban on corporate and union expenditures, a lower court and the FEC have interpreted the Citizens United ruling to mean that unlimited individual contributions must be allowed, clearing the way for “super PACs,” which are fundraising groups closely identified with candidates but technically independent.

The presidential campaign has seen an unprecedented explosion of spending by wealthy individuals, many making multimillion-dollar contributions on behalf of their favored candidates. Obama and other Democrats have pushed without success for new disclosure rules and other measures in response.

The political committee and corporation that challenged the Montana law argued that the Supreme Court should simply overturn the state court’s 5-to-2 decision, which acknowledged the apparent conflict with the Citizens United ruling.

Wrote one dissenting Montana justice, “When the highest court in the country has spoken clearly on a matter of federal constitutional law . . . this court is not at liberty to disregard or parse that decision in order to uphold a state law that, while politically popular, is clearly at odds with the Supreme Court’s decision.”

Montana Attorney General Steve Bullock told the court that states should have leeway in unique situations to regulate spending by corporate interests.

“It is a sad day for our democracy and for those of us who still want to believe that the United States Supreme Court is anything more than another political body in Washington, D.C.,” he said in a statement Monday.

The case is American Tradition Partnership v. Bullock .