Money, and Speech
I have tried to make this point a few different ways, all probably too colored with arrogance to be effective.
So I'll try to tone it down more.
There is nothing controversial about the statement that political donations are, as a category, protected to some substansial degree under the 1st Amendment.
And any (successful) challenge to Citizens United or McCutcheson must necessarily be found within that truth.
And political activity is not merely an expressive right. It also touches on freedom of assembly and association, freedom to petition government, to seek redress of grievances, and even religious freedoms where applicable.
If all of that were not true then Congress could out-law the Democratic Party itself. The very existence of organized political entities arises from the freedom of people to form political entities.
That does not mean that everyone can do everything with politics and money without limit. This is because politics is constitutionally deeper than expression.
The Constitution is not about how to make art or how to determine whose God is best. It is about keep your hands off art and religion!
But the Constitution is expressly about creating a political system. This is why political action is (or should be) regulate-able despite being protected. Running a democracy is even more basic to the Constitution than the Bill of Rights.
For instance, I would like to express my views by voting more than once. That is a clear instance of my political expressive rights running into the brick wall of the Constitution's deep purpose to create a functioning democracy.
Unlike art and religion, the conduct of elections actually is the Constitution's business... and also protected by rights. Thus there is a conflict. A tension.
But claiming that the First Amendment doesn't apply to funding political activity is a dead end, and not something anyone would actually want to be the case.
The dissenting justices in McCutcheon are smart people. Nowhere in their dissent did they say that political contribution is categorically outside the reach of the 1st Amendment.
All the justices agree that "money is speech" in the very limited sense that using money to promote a point of view is promotion of a point of view.
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The question in McCuthcheon is whether Congress has the power to limit the aggregate amount an individual gives to candidates under Congress' existing, established power to regulate contributions in order to prevent political corruption.
Taking the decision and the dissent together, all nine justices agree that "money is speech" (in certain contexts and for certain purposes) and all nine justices agree that Congress has an existing authority to regulate campaign donations.
All nine agree on both points.
So the debate, and it is a real and fierce debate, occurs within the overlap between those two things that all parties agree on.
(What the justices think in their heads is not the same as what they argue on paper. Whatever Scalia, for instance, might *think,* his legal position as a USSC Justice is defined by the decisions themselves. So professionally, Scalia admits to the existence of Congressional authority to limit campaign donations, despite the 1st Amendments general protection of such activities.)
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Is limitation of aggregate contribution within Congress' existing power to regulate donations for the purpose of shielding the system from corruption?
I think it is within Congress' existing power, so I would have voted with the four more liberal justices.