Washington ? Overturning a century-old restriction, the Supreme Court ruled Thursday that corporations may spend as much as they want to sway voters in federal elections.

In a landmark 5-4 decision, the court’s conservative bloc said corporations have the same right to free speech as individuals and, for that reason, the government may not stop corporations from spending to help their favored candidates.

The ruling — which will presumably apply to labor unions and other organizations — is likely to have an immediate effect on this year’s congressional elections.

Many political analysts and election-law experts predict millions of extra dollars will flood into this fall’s contests, much of it benefitting Republican candidates.

While Republicans praised the decision as a victory for wide-open political speech, Democrats slammed it as a win for big money.

President Barack Obama called the ruling “a major victory for big oil, Wall Street banks, health insurance companies and other powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans.” He promised to seek “a forceful response to this decision” from Congress. Some Democrats talked about seeking legislation requiring corporations to get approval from their shareholders before spending money on politics.

Sen. Mitch McConnell of Kentucky, the Senate Republican leader, said the court had restored proper rights to corporations and unions. Previously, “the government was picking winners and losers,” McConnell said.

Until Thursday, corporations and unions were barred from spending their own treasury funds on broadcast ads, campaign workers or billboards that urge the election or defeat of a federal candidate.

This restriction dates back to 1907 when President Theodore Roosevelt persuaded Congress to forbid corporations, railroads and national banks from putting money into federal races. After World War II, Congress extended this ban to labor unions. More recently, the McCain-Feingold Act in 2002 added an extra limit on corporate and union-funded broadcast ads in the month before an election. They were prohibited if they simply mentioned a candidate running for office.

Thursday’s decision swept away all these restrictions.