By a 5-4 decision, the Supreme Court on Thursday rolled back restrictions on corporate spending in federal campaigns. The decision could unleash a torrent of corporate-funded attack ads in upcoming elections.

"Because speech is an essential mechanism of democracy -- it is the means to hold officials accountable to the people -- political speech must prevail against laws that would suppress it by design or inadvertence," wrote Justice Anthony Kennedy for the majority.

In his dissent, Justice John Paul Stevens accused the majority of judicial activism and attacked the use of corporate personhood in the case: "The conceit that corporations must be treated identically to natural persons in the political sphere is not only inaccurate but also inadequate to justify the Court's disposition of this case."

Republicans offered measured praise for the decision, but progressive good-government groups and Democrats responded angrily and vowed to fight back with legislation.

"With its ruling today, the Supreme Court has given a green light to a new stampede of special interest money in our politics," said President Obama in a statement. "It is a major victory for big oil, Wall Street banks, health insurance companies and the other powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans... That's why I am instructing my Administration to get to work immediately with Congress on this issue. We are going to talk with bipartisan Congressional leaders to develop a forceful response to this decision."

Democracy 21's Fred Wertheimer, for years a leading advocate of campaign finance reform, called the decision a "disaster for the American people and a dark day for the Supreme Court."

"The Supreme Court majority has acted recklessly to free up corporations to use their immense, aggregate corporate wealth to flood federal elections and buy government influence. The Fortune 100 companies alone had combined revenues of $13 trillion and profits of $605 billion during the last election cycle," Wertheimer wrote.

"Under today's decision, insurance companies, banks, drug companies, energy companies and the like will be free to each spend $5 million, $10 million or more of corporate funds to elect or defeat a federal candidate -- and thereby to buy influence over the candidate's positions on issues of economic importance to the companies."

"We are moving to an age where we won't have the senator from Arkansas or the congressman from North Carolina, but the senator from Wal-Mart and the congressman from Bank of America," said Melanie Sloan, director of Citizens for Responsibility and Ethics in Washington.

The court found that the Federal Elections Commission overstepped its constitutional authority when it barred a conservative group called Citizens United from running ads for a movie attacking Hillary Clinton during the 2008 election season. Corporations and labor unions are now free to advertise -- and tell people to vote for individual candidates -- as they please. Before Thursday, corporations had to funnel money through political action committees to pay for ads, with limits on what could be spent. The court upheld, however, disclosure requirements for corporations that spend $10,000 to produce election-season ads, and ads will still have to disclaim who paid for them.

"Today's decision by the Supreme Court in Citizens United v. FEC, serves as an affirmation of the constitutional rights provided to Americans under the first amendment," said Republican National Committee Chairman Michael Steele in a statement. But, while Steele said he was pleased with the decision, he cautioned that unlimited spending by corporations will hurt the party apparatus.

"Free speech strengthens our democracy. While the Court's recognition that organizations have the freedom to speak on public issues and have their views protected from censorship is fundamental, the Court has now left an imbalance that disadvantages national parties in their ability to support their candidates."

Brad Smith, chairman of the Center for Competitive Politics, a group that filed an amicus brief in the case and since 2005 has advocated against campaign spending limits, praised the decision during a conference call with reporters.

"Most of us think that's good thing," he said. "Speech is important and this will be good in allowing unions and corporations to speak."

Labor unions don't seem to share the feeling.

"Unlimited corporate spending in federal elections threatens to drown out the voices of the people who should really be at the center of the political process, i.e., voters and candidates," said Anna Burger, treasurer of the Service Employees International Union, in a statement. "Unleashing corporate spending will only serve to distort and ultimately delegitimize the electoral process."

"It is a sweeping opinion -- in one fell swoop the Supreme Court struck down seemingly all bans on expenditures," said Marc Elias, a lawyer for congressional Democrats, in an interview with HuffPost. "We've had a series of Supreme Court rulings in recent years where the court splits the difference on certain things. We didn't see that here. They only thing they upheld was the disclosure provisions."

Elias is reportedly working with Democratic leadership to craft a legislative response, though he declined to provide any specifics.

"We'll wait and see whether or not the Hill decides to take this up," he said. "The court opinion notes that it looks to Congress in making findings about the need for campaign finance regulation so we'll see what the Hill does."

Sen. Robert Menendez (D-N.J.), chairman of the Democratic Senatorial Campaign Committee, said Democrats would look at a legislative fix. "Giving corporate interests an outsized role in our process will only mean citizens get heard less. We must look at legislative ways to make sure the ledger is not tipped so far for corporate interests that citizens voices are drowned out."

Sen. Chuck Schumer (D-N.Y.) added to the pile during a press conference: "The bottom line is this: the Supreme Court has just predetermined the winners of next November's elections. It won't be Republicans. It won't be Democrats. It will be corporate America."

One lawmaker has already launched a preemptive strike: Freshman Rep. Alan Grayson (D-Fla.) introduced a series of bills last week, such as the Business Should Mind Its Own Business Act and the Corporate Propaganda Sunshine Act.

Hans von Spakovsky, a legal fellow at the conservative Heritage Foundation, said he didn't think Congress would be able to undo the decision.

"I think Congress is going to have a very difficult time if they want to pass a law limiting individual expenditures," he said. "If you read the decision, the court is deciding on constitutional issues... It will be very difficult to design any kind of new federal statute that reimposes restrictions without it immediately being found unconstitutional again."

Here's video of Schumer's statement:

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Download a PDF of the decision here .