MADISON (WMSN) -- The United States Supreme Court overturned campaign finance laws dating back more than 100 years.



Justices ruled Thursday that corporations are like people, who have a free speech right to spend as much as they want to persuade people how to vote.



President Obama said it will unleash a stampede of special interest money. Watchdog groups worry it will breed more corruption.



It's arguably the most sweeping and consequential decision by the John Roberts-led court.



In a 5-4 split decision, justices ruled corporations and labor unions can spend their own treasury money on broadcast ads and billboards, supporting a candidate or a campaign issue.



"This does not allow corporations to give money directly to candidates, but to spend money on behalf of their own interests," said UW political science professor Charles Franklin.



The decision could have an immediate impact on this year's Wisconsin gubernatorial election. In 2006, candidates and special interests spent more than $32 million on the governor's race. Experts say, with fewer restrictions, that amount could skyrocket past $90 million come November.



"You're gonna see tons of advertising from corporate interests, running right up to election day," Franklin said.



Since 1907, when President Theodore Roosevelt called for campaign finance reforms, corporations and labor unions have had to create political action committees to raise election money. Now, they can dip into their own savings.



"For example, when you buy toothpaste now, that money can be used directly for television ads against people you believe in," said Sen. Russ Feingold (D-Wisconsin).



Feingold says the ruling destroys the bi-partisan reforms he and Sen. John McCain (R-Arizona) helped to create.



"This is an activist Supreme Court that's making its own laws," Feingold said. "They have in effect shown complete disrespect for the laws of this country."



"This disastrous decision paves the way for free and unlimited special-interest spending in our elections," said Sen. Charles Schumer (D-New York). "With the stroke of a pen the court decided to overturn hundred year old rule and override the millions of americans who want their voices heard in our democracy."



Even though conservative-leaning justices wrote the majority opinion, experts say both parties will benefit.



Corporations are more likely to support conservative candidates, labor unions are more likely to support liberals.



The real losers, experts say, will be the American people, most of whom can't compete financially with big business coffers.



Watchdog groups say if there's a positive outcome of this ruling, it's that it upholds disclosure laws, meaning companies do have to report how much money is spent and where it comes from.

Supreme Court overturns corporate limits on elections