President Obama on Saturday promised to fight a Supreme Court decision easing limits on political donations by corporations and unions, saying he couldn't "think of anything more devastating to the public interest."
In its 5-4 decision this week, the high court overturned two decisions and threw out parts of a 63-year-old law that said companies and unions can be prohibited from using their own money to produce and run campaign ads that urge the election or defeat of particular candidates by name.
The case involved a film by conservative group Citizens United, which criticized then-presidential candidate Hillary Clinton during the 2008 Democratic presidential primary campaign.
Supporters called the decision a big win for free speech.
But in his weekly radio and Internet message Obama said it was unacceptable.
"This ruling opens the floodgates for an unlimited amount of special interest money into our democracy," the president said. "It gives the special interest lobbyists new leverage to spend millions on advertising to persuade elected officials to vote their way -- or to punish those who don't."
Obama said that means public servants who stand up to Wall Street banks, oil companies, health insurers and other powerful interests could find themselves under attack when election time rolls around.
"The last thing we need to do is hand more influence to the lobbyists in Washington or more power to the special interests to tip the outcome of elections," he said.
Senate Republican Leader Mitch McConnell disagreed, calling the ruling a "monumental decision" that restores First Amendment rights to those who want to "express themselves about political candidates."
"Our democracy depends upon free speech, not just for some but for all," he said in a statement.
"This is a victory for Citizens United, but even more so for the First Amendment rights of all Americans," said Citizens United President David Bossie. "The fault line on this issue does not split liberals and conservatives or Republicans and Democrats. Instead, it pits entrenched establishment politicians against the very people whom they are elected to serve."
The court issued its ruling just as crucial midterm election campaigns are getting under way and as Obama's Democratic Party feels the pressure from a string of losses in New Jersey, Virginia and in Massachusetts, where this week Republican Scott Brown came from behind to win a Senate seat Democrats had held for decades.
Obama said the decision will make it harder to enact financial reforms, close tax loopholes, promote energy independence and protect patients from insurance company abuses -- key elements of his domestic agenda.
"We don't need to give any more voice to the powerful interests that already drown out the voices of everyday Americans," he said. "And we don't intend to."
He said he has instructed his administration to work with Congress to "fight for the American people" and develop a "forceful bipartisan response" to the decision.
"It will be a priority for us until we repair the damage that has been done," Obama said.
The Associated Press contributed to this report.