- I decide to speak in favor of a politician or cause in which I believe.
- I decide to buy ad space/time on TV, Radio, or Print media to favor a cause I believe.
- I, and some friends who agree with me, decide to pool our resources to buy more time/space, or time/space in larger (or more) markets than we could do individually.
- I, and some friends, decide to donate our pooled resources to someone else to do the kind of political activity I describe above on our behalf.
- I, and some friends, decide to solicit money from others to do the same stuff I've just described.
- I, and some friends, decide to organize into an association to do a variety of things, part of which is to make the kinds of political speech I just described.
- I, and some friends decide to "sell" part "ownership" of the association which does a variety of things, including the political activity associated above and give them a vote, weighted on the amount they've put into the "pool" for the association, for folk to manage how those pooled resources are used.
People engage in speech, not organizations. When you ban an organization from engaging in political speech, including the speech of buying ad space and the like, you aren't stopping some faceless entity, but the people of that organization.
I would say the same thing (and do, in fact) if instead of "corporations" the argument was about unions.