How did the McCutcheon majority distinguish political access from corruption?

In McCutcheon v. Federal Election Commission (2014), the Supreme Court majority, led by Chief Justice John G. Roberts, Jr., distinguished political access from corruption by narrowly defining the latter as quid pro quo corruption—the direct exchange of an official act for money, or bribery.1

The Court held that the government’s only legitimate interest in restricting campaign contributions is the prevention of this specific type of corruption. Conversely, the majority argued that increased access to or influence over an officeholder, as well as the act of ingratiating a donor to a candidate, do not constitute corruption.1

According to the ruling, these forms of engagement are a "central feature of democracy," as they reflect the expectation that elected officials will be responsive to the concerns of constituents who share their beliefs and interests. Consequently, the Court concluded that the government cannot prohibit contributions based on the desire to limit such access or ingratiation without "impermissibly injecting the Government into the debate over who should govern".1