A case the Supreme Court is to take up this morning over the legal treatment of wealthy, self-funding congressional candidates has the potential to reshape campaign finance law far beyond the narrow issue of the so-called millionaire’s amendment, lawyers on both sides of the dispute said.

The justices are considering a provision in the 2002 Bipartisan Campaign Reform Act that raises contribution limits when candidates face a self-funding opponent. The lawsuit going before the court was brought by a businessman who has made two unsuccessful bids for Congress and is planning another campaign this year, John Davis of the town of Clarence in Western New York. He contends that the self-funding-related rules chill his First Amendment rights by discouraging him from any spending which would unleash the increased limits, effectively flooding his opponent’s campaign with money.

“I’m not sure the court grasps what is potentially at stake here. In fact, I’m not sure if the parties in the case do,” a former chairman of the Federal Election Commission, Bradley Smith, said.

Since the seminal case of Buckley v. Valeo was decided in 1976, the main basis for upholding campaign finance regulation has been a desire to avoid both actual corruption and the appearance of corruption that can occur when large sums of money flow into political campaigns. The millionaire’s amendment doesn’t seem to advance either of those goals, but rather to level the playing field by ensuring that candidates are not massively outspent by an opponent.

Mr. Smith said that if the Supreme Court accepts that interest as a legitimate one for the government to pursue, proponents of tight regulation and of public financing will have scored a major victory. “If they win this, it gives them a bigger win than they ever got in McConnell,” the former FEC member said, referring to the 2003 Supreme Court decision which upheld most of the 2002 campaign finance overhaul, popularly known as McCain-Feingold after its Senate sponsors. “The court would basically be saying, ‘It’s okay for the government to put its thumb on the scale in this way,'” Mr. Smith said.

Most of the usual players in the legal battles over campaign finance have lined up in their expected positions. Groups such as the Cato Institute and one Mr. Smith founded, the Center for Competitive Politics, have filed briefs urging the court to strike down the millionaire’s amendment. Common Cause, the Campaign Legal Center and the Brennan Center at New York University are among those arguing to preserve the rule, though some sound less than enthusiastic about the effort. “This wasn’t a big goal of campaign finance reformers, the millionaire’s amendment,” Tara Malloy of the Campaign Legal Center said.

The provision, which was not in the original legislation from Senators McCain and Feingold, was added in the Senate. Mr. McCain is the presumptive Republican nominee for president.

Critics have called the amendment an incumbency protection measure, but Ms. Malloy said those claims are “really exaggerated.” She pointed to statistics showing that out of 110 cases where the amendment was triggered, only six of those who stood to benefit were incumbents.

Ms. Malloy said so-called reform groups are mostly concerned about “side issues” the Court could get into. “We hope the court upholds the millionaire’s amendment, but it’s important that the court attends only to that issue and doesn’t go too far astray,” she said.

Under the law, when House candidates spend more than $350,000 of their own funds on a campaign, others running for the same House seat can accept donations of $6,900 a person, or three times the normal $2,300 limit. When a Senate seat is at stake, those facing a self-funding candidate can raise as much as $13,800 from each donor, depending on how much the self-funding candidate exceeds the limit, which varies depending on the size of the state.

Under the legislation, state and national political parties can also make unlimited donations to all House candidates facing someone who has exceeded the cap and to Senate candidates facing opponents whose spending is greater than a figure calculated by a formula.

One quirk of the system is that once the millionaire’s amendment kicks in, donors can still give no more than $2,300 to the self-funding candidate, even when the opposition is taking donations three or six times that size. It’s unclear whether Mr. Davis has standing to raise that argument on behalf of donors who would regard that as unfair, but the disparity was pointed out in an amicus brief a Stanford legal clinic filed on behalf of two candidates fined for failing to report their self-funding promptly. The case has forced both sides of the campaign finance debate into contorted positions at odds with their usual arguments. Those who support tight controls on money in politics and who warn about the corrupting influence of money are actually backing more lax contribution limits for some. And those who believe in virtually no controls on campaign funds are effectively asking for strict and uniform enforcement of the standard $2,300 limit.

The main task of defending the law today is expected to fall to the Solicitor General, Paul Clement. Mr. Davis is represented by Andrew Herman of Brand Law Group in Washington.