Washington ? The Supreme Court Wednesday upheld key provisions of last year’s campaign finance overhaul law that seek to curb the influence of wealthy big-bucks contributors on the political system.

The court ruled that Congress did not violate the Constitution when it enacted a ban on unlimited “soft money” donations to the political parties and placed strict restrictions on the financing of political TV commercials just before an election.

In a 5-4 ruling, the court held that these provisions did not violate the First Amendment’s guarantee of free speech because the government has a legitimate interest in “preventing the actual or apparent corruption of federal candidates and officeholders.”

The dissenting justices argued that the law represented an ominous first step toward greater government restrictions on free speech. “The court’s willingness to impute corruption … greatly infringes associational rights and expands Congress’ ability to regulate political speech,” wrote Chief Justice William Rehnquist.

The decision was a major victory for a coalition of good-government groups and their congressional allies, who have campaigned for years to purge soft money from the system. “It was a grand slam for us,” said Rep. Chris Shays, R-Conn., one of four co-sponsors of the law. “It basically means that you no longer are going to have these large checks from wealthy donors or these unlimited contributions from the treasuries of corporations or the dues of unions.”

But the ultimate impact of the law remains uncertain, in large part because wealthy individuals with political interests already are exploring ways to circumvent the soft money ban. Some have recently formed so-called 527 groups — so named because they are formed under rules outlined in Section 527 of the Internal Revenue Code — to raise and spend large amounts of money on behalf of political causes.

Indeed, the court’s majority opinion, co-authored by Justices John Paul Stevens and Sandra Day O’Connor, frankly acknowledged that the law in question was hardly airtight. “Money, like water, will always find an outlet,” they wrote. “What problems will arise, and how Congress will respond, are concerns for another day.”

The partisan political impact of the newly upheld law is also likely to play out in complex and perverse ways. In the short run, most observers say it will work to the disadvantage of the Democrats because they have been far less successful than Republicans in developing a wide base of small donors willing to sustain the party’s activities. In recent campaigns, Democrats routinely used the now-banned soft money mega-donations to narrow the fund raising gap with their rivals.

But Democratic officials say they have made a good beginning in developing a small-contributor base. And one Democratic presidential candidate, former Vermont Gov. Howard Dean, has achieved something of a breakthrough in that area this year by using the Internet to raise millions of dollars in small amounts from hundreds of thousands of contributors.

The law upheld Wednesday prohibits all contributions to parties from corporate or union treasuries and sets a $25,000 annual limit on donations from an individual to a single party committee. It also sets a limit of $57,500 on the total amount any individual can give in a two-year period to all party committees and political action committees.

Before the law took effect late last year, political parties had been free to accept contributions of unlimited size from virtually any source. These soft money donations were supposed to be used only for get-out-the-vote activities and general party promotion, but both parties routinely circumvented that provision and used soft money to finance sophisticated TV ads promoting their candidates.