The Supreme Court is set to hear Shaun McCutcheon v. Federal Election Commission, a case that could roll back campaign finance restrictions to those before the Watergate era. (Thomas LeGro/The Washington Post)

The very wealthy could play a much greater role in funding federal candidates and political parties if the Supreme Court rules that a key campaign finance restriction adopted after Watergate is unconstitutional.

Under Chief Justice John G. Roberts Jr., the court already has junked a number of election spending limits as improper restrictions on political expression — perhaps most dramatically with its 2010 Citizens United decision, which wiped out the ban on corporate election spending.

A bold and broad decision by the court in one of its first cases of the new term, Shaun McCutcheon v. Federal Election Commission, which the justices are to hear Tuesday, could overturn decades of precedent about the remaining power the government has to limit contributions to candidates and parties.

Critics argue that such limits impinge free expression. Supporters view the caps as the last dam preventing a tidal wave of money from flooding federal campaigns, which have been buffeted by an explosion of super PACs fueled by unlimited donations.

The current case, brought by Shaun McCutcheon, a businessman from Hoover, Ala., and the Republican National Committee, involves a restriction that many Americans are probably unaware of and even fewer could afford to violate: the limit on the overall amount that one person may give during a two-year election cycle to federal candidates, political parties and committees.

That limit is $123,200, including a $48,600 cap on total candidate contributions. If the court sides with McCutcheon, party leaders could set up a joint fundraising committee with their presidential nominee, congressional candidates and state affiliates to accept nearly $3.7 million from an individual in each election cycle, defenders of the limits say.

“A system in which an individual can provide millions of dollars — potentially in response to direct solicitations from the president and members of Congress — to finance parties and their candidates would substantially replicate the Watergate-era and soft-money systems that resulted in well-documented instances of corruption and apparent corruption,” Solicitor General Donald B. Verrilli Jr. told the court in a brief.

Advocates of campaign finance regulation say the stakes are much higher than simply preserving the current system of aggregate limits.

“They very well may set the stage to strike all contribution limits,” said Fred Wertheimer, president of Democracy 21, a group that seeks to reduce the influence of money on politics. “That would take us back to the 1870s and the era of robber barons.”

McCutcheon, the chief executive of an electrical engineering firm, favors tea party candidates and laughs off descriptions of himself as “Alabama’s Koch brother,” a reference to Charles and David Koch, the billionaire industrialists who back conservative causes.

“I’m working on my first billion and I do believe in the freedom to do that,” McCutcheon said at a recent lunch interview in Washington. “I’m just another political activist trying to change the world.”

As a donor, McCutcheon is a relative newcomer on the national scene. He started with a handful of small contributions to the National Republican Senatorial Committee in 2008, then began giving in earnest to GOP congressional challengers in the 2012 election cycle, doling out checks in patriotic $1,776 increments. In all, he donated nearly $66,000 to federal candidates and political committees in the last cycle.

He would have given more, McCutcheon said, but was startled to learn that there was a limit on how much he could contribute. Spurred on by campaign finance lawyer Dan Backer, whom he met at a gathering of conservative activists, McCutcheon said he decided to challenge the law.

“I’ve always understood base limits,” he said. “It’s the aggregate limits that were a surprise.

“The whole thing is an important First Amendment free-speech thing,” McCutcheon added. “It’s about your right to spend your money however you choose on as many candidates as you choose. It’s freedom.”

At the heart of the case is the framework created by the court’s seminal 1976 Buckley v. Valeo decision, which upheld limits on campaign contributions that Congress put in place two years earlier in response to the Watergate scandal. That ruling drew a distinction between contributions, which the court said could be limited to prevent corruption or the appearance of corruption, and expenditures, which the court determined were a form of direct personal expression.

That decision led to the current lopsided campaign system, in which donors can give a federal candidate only $2,600 per election, but can donate endless sums to super PACs, which must spend their money independently of candidates and parties.

The court may now consider whether the line it drew between contributions and expenditures still makes sense.

“It is not hard to see a situation in which they could really soften up the boundaries,” said Robert Bauer, a Democratic lawyer who specializes in election law. “It could be a step in the direction of weakening the Buckley framework.”

That is the hope of those who want to roll back campaign finance regulations, who argue that the reasoning behind contribution limits no longer holds in a super PAC environment.

“Base limits need to be much larger, if they survive at all,” said James Bopp Jr., a campaign finance lawyer representing the RNC in the case. “Nobody thinks you can buy even a Democratic congressman for $2,600.”

Although McCutcheon is not challenging base contribution limits, Senate Minority Leader Mitch McConnell (R- Ky.), a longtime foe of campaign finance restrictions, is using the case to make that broader argument. In a relatively rare move, the court has given McConnell’s lawyer time to present that case at Tuesday’s oral arguments.

Although both political parties attract deep-pocketed donors who are willing to contribute millions of dollars, they diverge sharply on the merits of campaign finance limits, with Democrats largely trying to protect such regulations and Republicans seeking to dispose of them.

It is the same on the court, where the five Republican-nominated justices are the ones most likely to believe that such regulations threaten political speech.

Justices Antonin Scalia, Anthony M. Kennedy and Clarence Thomas have made clear that they are ready to overturn Buckley, leaving the decision in the current case most likely in the hands of Roberts and Justice Samuel A. Alito Jr.

Paul D. Clement, who defended campaign finance laws when he was President George W. Bush’s solicitor general, said he thinks that Roberts, who often favors a more incremental approach on the court, is the least likely of those to want to overturn Buckley. But it is not necessary to do that to decide the case in favor of McCutcheon, he said.

“It’s almost as if Congress’s own per-candidate limits can be used against the government,” Clement said. Because McCutcheon is giving less money to each candidate than what Congress said is a corruption concern, what difference does it make how many candidates he helps?

For his part, the Alabama businessman think his case could be a boon to Democrats and Republicans alike.

“It will put more money into politics, that’s for sure,” McCutcheon said. “What’s wrong with spending all the money you want to spend on that? My mom would tell me not to do that, and she’s probably right, but the government shouldn’t tell me not to do that.”

Alice Crites contributed to this report.