The Supreme Court's decision Thursday striking down some limits on corporate and labor spending in elections is a "terrible mistake," Sen. Russ Feingold (D-Wis.) said.

Feingold, the sponsor of a 2002 campaign finance reform law with Sen. John McCain (R-Ariz.), expressed relief that aspects of that law were left intact, but said the court's ruling upended years of precedent.

"This decision was a terrible mistake," Feingold said in a statement, promising to pass legislation to address the ruling.

"Just six years ago, the court said that the prohibition on corporations and unions dipping into their treasuries to influence campaigns was ‘firmly embedded in our law,’" Feingold added. "Yet this court has just upended that prohibition, and a century's worth of campaign finance law designed to stem corruption in government."

The 5-4 decision, written by Justice Anthony Kennedy, will free up corporations, labor unions and other groups to spend more heavily in elections. The ruling is especially significant for 2010, during which critical congressional midterm elections will be held in November.

Feingold expressed relief that the court's ruling leaves in place restrictions on so-called "soft money" donations in elections, but said the broader decision would prompt work on legislation.

"The American people will pay dearly for this decision when, more than ever, their voices are drowned out by corporate spending in our federal elections," the Wisconsin Democrat said. "In the coming weeks, I will work with my colleagues to pass legislation restoring as many of the critical restraints on corporate control of our elections as possible."