Why did Chief Justice Roberts say aggregate limits did not prevent quid pro quo corruption?

In the case of McCutcheon v. Federal Election Commission, Chief Justice John G. Roberts, Jr. argued that aggregate contribution limits did not effectively prevent quid pro quo corruption—defined as the direct exchange of an official act for money—for several reasons.1

Roberts maintained that the only legitimate governmental interest for restricting campaign finances is the prevention of actual or apparent quid pro quo corruption. He asserted that aggregate limits failed to serve this function in any meaningful way because they did not prevent the circumvention of "base limits" (the limits on contributions to individual candidates).1

He dismissed hypothetical scenarios of circumvention as "implausible" or "divorced from reality," noting that current laws and regulations—such as earmarking rules and base limits on contributions to political action committees (PACs) and party committees—already make it difficult, if not impossible, for a donor to funnel large sums to a single candidate. Furthermore, Roberts argued that if a base-limit contribution to one candidate does not create a risk of corruption, then a similar contribution to a tenth or eleventh candidate cannot be considered corruptible.12