Alabama businessman Shaun McCutcheon and his GOP allies insist their Supreme Court challenge to a cap on overall campaign contributions in one election cycle doesn’t dispute the constitutionality of the “base” limit on how much an individual can give to a single candidate in a single election.

“This case is not about base limits; they make sense,” said McCutcheon, whose challenge in McCutcheon v. Federal Election Commission is scheduled for oral argument before the Supreme Court on Oct. 8. “The corruption argument on base limits is pretty solid. If you were running for Congress and I gave you $1 million, wouldn’t you owe me?”

The Republican National Committee has joined McCutcheon in arguing that the aggregate limits muzzle free speech. Indiana lawyer James Bopp Jr., who is representing the RNC in the case, also stresses: “We’re not challenging base limits in this case.”

The case is shaping up as a key test of how far this high court is willing to deregulate the campaign finance system.

But it’s easy to see why McCutcheon’s opponents warn that his challenge poses a direct danger to contribution limits of all kinds. The aggregate limits now bar an individual from giving more than $48,600 to candidates in one election cycle, and more than $74,600 to political action committees and parties. The cap on base contributions to a single candidate is much lower, at $2,600 for individuals, and is seen as one of the last remaining pillars of the campaign finance regime.

The McCutcheon case threatens the underlying limits on two fronts, advocates of campaign finance restrictions say. First, knocking down the aggregate limits would give big donors an easy way to get around the underlying cap on direct candidate donations. Candidate-party joint fundraising committees already collect checks into the tens of thousands of dollars from big donors. Without the aggregate limits, those checks might soar into the millions.

The McCutcheon challenge also may prompt the high court to regard contribution limits with a more critical eye and to hold them to a more rigorous standard of constitutional scrutiny. In its landmark Buckley v. Valeo ruling in 1976, the high court held that limits on campaign spending were constitutionally suspect, while caps on direct contributions to candidates were justifiable to prevent corruption and its appearance.

This distinction between expenditures and contributions helps explain why, in 2010, the Supreme Court in Citizens United v. FEC knocked down long-standing limits on independent campaign spending by non-party outside groups.

In McCutcheon v. FEC, both the RNC and Senate Minority Leader Mitch McConnell, R-Ky., who has been given special permission to join the Oct. 8 oral argument, argue that the Supreme Court should throw out that Buckley distinction between expenditures and contributions.

“This case represents an opportunity for the Court to reject, based on almost four decades of experience, the less rigorous, ‘complaisant’ level of First Amendment review accorded to contribution limits,” McConnell states in his amicus brief.

Even if the McCutcheon case doesn’t take on the underlying contribution limits, it’s clear that such a challenge is not far off. In the wake of Citizens United, a growing chorus of both federal and state lawmakers complain that they now face more stringent limits than unrestricted outside groups such as super PACs.

Half a dozen states including Arizona, Maryland and Wyoming have eased restrictions on contribution limits since 2010, and several more are considering similar changes. The Citizens United ruling “emboldened opponents of campaign finance reform,” noted Elizabeth Wydra, chief counsel for the Constitutional Accountability Center. “So I think there are people who want to do away with any and all limits on political contributions.”

In previous court rulings, Supreme Court Justices Antonin Scalia, Anthony M. Kennedy and Clarence Thomas have all voiced skepticism that contributions should be held to a more-relaxed standard of scrutiny than expenditures.

“Buckley’s dichotomy between contributions and expenditures is highly controversial, and we know at least three justices have rejected it,” said Bopp, who makes no bones about his disdain for the base contribution limits.

“The reality is that contribution limits are marginalizing candidates, PACs and parties in particular,” he added, “because it forces money to go other places.”

True, the Supreme Court has consistently rejected past challenges to the contribution limits. But even if base limits remain constitutional, a McCutcheon win could make the high court more receptive to arguments that they are too low and should be raised. Little wonder that McCutcheon’s opponents have set out to discredit his challenge in the strongest possible terms.

If the Supreme Court overturns aggregate limits, argues Democracy 21 President Fred Wertheimer, “not only do you create a system of legalized corruption, but you open the door to all of the contribution limits being struck down.”