The law allows candidates to receive larger campaign contributions when their wealthy opponents spend heavily out of their own pockets.

The court said by a 5-4 vote that the law violates the First Amendment.

The law was challenged by Jack Davis, a New York Democrat who has so far spent nearly $4 million of his own money in two losing campaigns for Congress and says he will spend another $3 million this year.

Davis says the provision in 6-year-old campaign finance reforms unfairly rewards his opponents by letting them exceed campaign fundraising limits simply because Davis dipped into personal funds.

Writing for the majority, Justice Samuel Alito said that under the amendment, the vigorous exercise of the right to use personal funds to finance campaign speech produces fundraising advantages for the opponents of wealthy candidates.

Alito said that if the millionaire's amendment raised the contribution limits for all candidates, Davis's challenge to the law "would plainly fail," raising the question of whether Congress could easily fix what the Supreme Court struck down.

The amendment has come into play in relatively few races. Its most prominent beneficiary so far has been Sen. Barack Obama. He was able to attract additional contributions for his Democratic senatorial primary campaign in Illinois because an opponent spent nearly $29 million of his own money.

A co-author of the 2002 campaign finance law, Sen. Russ Feingold, D-Wis., said the Supreme Court decision has no impact on the central component of the reforms, the ban on six-figure political donations to political parties. Feingold co-authored the reforms with Sen. John McCain, the Republican presidential nominee-in-waiting.

Sen. Charles Schumer, D-N.Y., said the court had issued a "confounding decision that takes the First Amendment to an illogical, distorted extreme."

Davis lost in 2004 and 2006 to Republican Rep. Tom Reynolds, who spent more than $5 million in winning re-election two years ago, 51 percent to 49 percent.

Reynolds chose not to solicit increased contributions after Davis triggered the millionaire's amendment by putting at least $350,000 of his own money into the race. Reynolds could have received $6,900 from individual donors, triple the limit otherwise. Reynolds is retiring at the end of this term.