CAMBRIDGE, Mass. — ON Tuesday the Supreme Court is scheduled to hear arguments in McCutcheon v. Federal Election Commission, potentially the most significant federal campaign finance case since Citizens United in 2010.

But while the court in Citizens United struck down — correctly, in my opinion — limits on independent campaign spending by individuals or organizations, the McCutcheon case is an attack on limits that should not be struck down: those on contributions made directly or indirectly to political candidates.

The McCutcheon case was brought by the Republican National Committee and a contributor, Shaun McCutcheon. If they succeed, individuals will be able, in effect, to direct unlimited amounts of cash to the election campaigns of federal candidates — inviting corruption or the appearance of corruption, which the Supreme Court has consistently held justifies contribution limits. (I have filed an amicus brief in this case on behalf of Americans for Campaign Reform.)

Campaign-finance law currently limits an individual’s contribution to a federal candidate to a modest sum — around $2,500 — with a somewhat larger limit for donations to committees that contribute to the candidate. In addition, the law places an aggregate limit — around $120,000 — on an individual’s contributions, direct or indirect, to all federal candidates every two-year election cycle.

Mr. McCutcheon and the R.N.C. say they object to the aggregate limit on contributions, not to the per candidate limit.

Ever since the 1976 Supreme Court case Buckley v. Valeo, in which the court upheld limits on individual federal campaign contributions, every Supreme Court decision on this issue has been based on the distinction between money given to candidates — contributions — and money that individuals or organizations use for their own independent campaign-related expenditures.

The underlying idea is that while the First Amendment prohibits the government from limiting your political speech (and the more you speak, the more you may have to spend), a contribution is money spent to help someone else speak. The government may not limit your own expression (and since Citizens United that applies to corporations and unions, too), but for almost half a century Congress has limited contributions without being challenged by the Supreme Court.

All federal campaign finance laws since 1972 — not just the current Bipartisan Campaign Reform Act signed into law in 2002 — have also had aggregate limits, which have always been upheld by the court.

Mr. McCutcheon and the R.N.C. contend that the aggregate limits on contributions violate the First Amendment by constraining a contributor’s opportunity for political expression. This is unconvincing: Mr. McCutcheon is already permitted to spend as much money as he would like on his own independent campaign-related speech.

To get the full measure of how far-fetched Mr. McCutcheon’s claim is, consider his argument that aggregate contribution limits violate his right to political expression by preventing him from contributing the symbolic amount of $1,776 to each of 25 candidates (along with a total of $7,500 to three others) — though he could give $17.76 to every Republican Congressional candidate. It is as if the developers of One World Trade Center, which is 1,776 feet high, had argued that the First Amendment gave them the constitutional right to build to that height.

Though Mr. McCutcheon and the R.N.C. claim that they are not attacking all contribution limits, only aggregate limits, they know that in practice that is a distinction without a difference. If the court were to strike down aggregate limits, then a contributor could give money up to the maximum amount not only to every federal candidate but also to any political action committee that contributed to a candidate. And as a lower federal court in McCutcheon recognized, there is no limit on how many PACs might be created that are likely to contribute to a particular candidate.

If there were 10,000 PACs, all of which were likely to contribute to John Smith for Congress, then by Mr. McCutcheon’s and the R.N.C.’s reasoning you should be free to give up to the maximum of $5,000 to each PAC, for a total of $50 million. This would blow an enormous hole in the longstanding, court-approved scheme of campaign contribution limits.

As Justice Antonin Scalia once wrote in another context, this argument is not a wolf in sheep’s clothing: “this wolf comes as a wolf.” The only reason the Supreme Court would be tempted to let this wolf in is if the court wants to see the destruction of all limits on an individual’s donations to a political candidate.