If it's a natural person, then they can do it with their own time and money. This is not what we've seen in the post-CU political financing world: we've increasingly seen opaque corporate structures where unknown individuals ply unknown amounts of money (collected from the labor of people who almost certainly wouldn't approve it directly) into races.
CU does nothing to protect the free expression of individuals; it has markedly diminished the expressive power of individuals in the political sphere in favor of opaque and legally established (rather than natural) entities.
> So you are only against Citizens United as applied to big and powerful corporations, but are totally fine with it with less powerful corporations? Say, you totally support the actual plaintiff, the Citizens United organization, in its right to publish the movie that was the subject matter in the case?
I don't know if this is intentional on your part, but you're dropping a key piece of context: Citizens United (the organization) is a PAC, with extraordinarily wealthy corporate financiers. It's not some kind of scrappy outfit with a handful of unpaid undergraduate interns, and it would not exist independent of the corporate interests that use it as a more palatable front for political influence ("Koch Interests Action Campaign" just doesn't have quite the same ring to it, I think).
> Should government also have a right to restrict speech of individuals if they are rich and powerful enough?
Frankly, there should be no private financing of elections at all. But that's not realistic.
Realistically: rich and powerful people have the exact same right to engage in electoral politics as everyone else, and there is no immediately feasible way to stop them from dominating those politics via their wealth and power. What we can do is make those attempts at domination as transparent as possible. Rulings like CU directly stand in the way of electoral transparency.