Why did Justice Breyer argue that the McCutcheon decision weakened campaign-finance laws?

In his dissenting opinion in McCutcheon v. Federal Election Commission, Justice Stephen Breyer argued that the majority’s decision "eviscerates" campaign finance laws, leaving them unable to address significant threats to democratic legitimacy.1

Breyer’s critique centered on several key points:

• Narrow Definition of Corruption: He contended that the majority’s focus on "quid pro quo" corruption—a direct exchange of money for political favors—was unrealistic and inconsistent with Supreme Court precedents established since Buckley v. Valeo.1
• Circumvention of Base Limits: Breyer disputed the majority's claim that removing aggregate limits would not lead to the circumvention of "base limits" (the caps on contributions to individual candidates). He provided specific scenarios demonstrating how these limits could be bypassed and argued that the majority failed to prove that alternative measures would be as effective as aggregate limits.1
• Judicial Overreach: He faulted the plurality for reversing the lower court's decision without returning the case for evidentiary proceedings, arguing that the Court lacked the expertise to dismiss the factual risks of corruption without a proper empirical record.1