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Citizens United sued, and the decision has been steadily moving up the federal courts for the past two years.

President Obama also promised to try to curb the decision.

“With its ruling today, the Supreme Court has given a green light to a new stampede of special-interest money in our politics,” the president said 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.”

Richard Briffault, a law professor at Columbia Law School, said it will be difficult for Congress to craft legislation to limit business spending, since most corporate law is written at the state level.

He also said it’s not immediately clear how much will change in campaign finance, but expressed doubt about the dire predictions. He noted that about half of states had banned corporations from making independent political expenditures and half had allowed it.

“I don’t think you could see one was demonstrably more politically free,” he said.

Thursday’s ruling does strike down state laws that banned independent corporate political spending.

Those who fought for more campaign freedom cheered the ruling.

“The Supreme Court has restored a part of the First Amendment that had been unfortunately stolen by Congress and a previously wrongly decided ruling of the court,” said Hans A. von Spakovsky, a former Republican member of the Federal Election Commission and current senior legal fellow at the Heritage Foundation.

The Supreme Court first heard the case, Citizens United v. the Federal Election Commission, in its 2008-09 term, but instead of issuing a ruling, the court asked that the case be reheard to address the fundamental questions of restrictions on corporations.

The case was reheard in September and was the first in which Justice Sonia Sotomayor heard oral arguments. She sided with the court’s minority in Thursday’s decision.

In the wake of the ruling, a number of groups erroneously accused the court of lifting the limits on corporate contributions to political candidates. The decision does not end those limits, which require any contribution to be made by a corporation’s political action committee, which must abide by strict rules.

“The opinion very specifically talks about and upholds the limits on contributions,” Mr. von Spakovsky said.

The decision also doesn’t overturn the ban on so-called “soft money,” the uncapped donations to political parties that had swamped the political process in the 1990s.

The court, ruling 8-1, also upheld the law’s requirements that any group running political ads must disclose the names of its contributors. Justice Thomas dissented from that part of the decision, arguing that there is a right to anonymous speech that is being circumscribed by the disclosure and reporting requirements.