WASHINGTON—Outside political organizations will play a larger role in the 2010 midterm congressional elections after a decision by the U.S. Supreme Court Thursday to strike down elements of the McCain-Feingold campaign-finance law.

"There will be a lot more groups advocating for and against candidates," said Cleta Mitchell, a lawyer with Foley & Lardner who advices outside political organizations. "It rips the duct tape off the mouths of the American people."

The Supreme Court decision stripped away rules that limited the ability of corporations, unions and other organizations to fund and organize their own political campaigns for or against candidates. The court also struck down a part of the McCain-Feingold campaign-finance law that prevented any independent political group from running advertisements with 30 days of a primary election or 60 days before a general election.

Together, the decisions make it easier for corporations, labor unions and other entities to mount political campaigns for and against candidates for Congress and the White House.

The Supreme Court decision is the latest in a string of judicial rulings that have chipped away at federal limits on the political activity of outside groups in elections. Last year, a federal court overturned rules imposed by the Federal Election Commission that made it more difficult for outside political groups to raise money for political advertisements.