In a decision that could have a profound impact on future elections, the court overturned a 20-year-old ruling that barred businesses from paying for campaign adverts. The decision in what is known as the Citizens United case will allow big businesses - such as those selling arms, drugs or insurance companies - that already wield influence through lobby groups to openly back favored candidates who support their interests.
The court also struck down part of a separate campaign finance bill that barred companies and trade unions from publishing political adverts in the closing days of election campaigns.
The ruling was swiftly condemned by liberal pressure groups such as the People for the American Way, which described it as "putting corporate power above individual rights".
"It is a long established principle that the government can act in the interest of democracy to prevent corporations from pouring in billions of dollars to unduly influence elections," the group said. "Given conservative rhetoric about deference to the democratically elected branches, today's ruling is stunning".
Republicans are expected to be the principal beneficiaries of the ruling, a further blow to some Democratic party candidates in November's midterm elections who are already feeling exposed after the Republican candidate, Scott Brown, won an upset victory in Massachusetts, taking the seat left vacant by the death of Democratic icon Edward (Ted) Kennedy.
Nineteen of the Democrats' remaining 59 seats in the Senate are up for re-election in November. Several incumbents told Politico that all of them are now considered vulnerable.
"Every state is now in play," said Barbara Boxer, who is facing a serious challenge for her California seat from a wealthy Republican, Carly Fiorina.
This will effectively end the voice of the middle class and poor. How can we possibly compete in a world where money contributed determines the outcome of legislation? What politician will have the will power to take the money and vote against the ones who supplied it? I have said for the longest time that who is placed on the Supreme Court is in most ways as important as who wins the Presidency. The Bush years have brought us to the brink of financial collapse and now have almost assuredly ended our democracy as we know it. His reign of terror will be known as the catalyst that destroyed a once great democracy unless we rise up and fight back.
The only way out of this mess is an amendment to the Constitution but who thinks getting that will even be possible with support necessary from Republicans? We all know they are the party of big business and this ruling today has said that the party with the most money has the loudest voice. I am truly sickened by this decision and afraid for what the future holds for this once great land.