WASHINGTON  The Supreme Court on Thursday struck down a law meant to level the financial playing field when rich candidates pay for their own political campaigns.

The 5-to-4 decision, legal experts said, was significant for rejecting the rationale behind the law, known as the “millionaire’s amendment,” and for confirming the court’s continuing skepticism about the constitutionality of campaign finance regulations.

“Supporters of reasonable campaign finance regulation are now zero for three in the Roberts court,” said Richard L. Hasen, a professor at Loyola Law School in Los Angeles. “This is a signal of what is to come. What could easily fall following this case are the longstanding limits on corporate and union spending in federal elections.”

The law at issue in Thursday’s decision imposed special rules in races with candidates who finance their own campaigns. Those candidates are required to disclose more information, and their opponents are allowed to raise more money.