Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
That's the 1st Amendment, lead item of the historic Bill of Rights.
It's based on this prohibition that U.S. courts have certified the underwriting of campaigns by corporations and broadcasting to a hundred million eyes and ears at once. But no basis is there. Go ahead and read it again.
Money does not speak, and neither does TV or radio. If a corporation or union could speak, it wouldn't need a spokesperson.
Speech is what comes from mouths, is made powerful by assembly, and is reproducible from products of the printing press when read aloud. As the founders would say, it's self-evident, or should be.