The Supreme Court's attribution of constitutional rights to corporations is unsupported by the U.S. Constitution or the writing of the Constitution's authors. The invention of corporate personhood was an act of raw judicial activism that undermines our Constitution's promise of a republican form of government.
While our Constitution's authors were alive, corporations were completely subordinate to democracy. They could not own stock, engage in activities other than those essential to their business, nor could they make any political or charitable contributions. Corporate lobbying also was prohibited. Maybe they were onto something?
No human being's First Amendment rights will be infringed by preventing corporations from engaging in electoral advocacy. Though we dissent from the Supreme Court's view that spending money to influence election outcomes is equivalent to speech, every corporate executive remains free to spend unlimited amounts of his/her own money to advocate for or against a political candidate or party (only direct investments to a candidate's campaign fund are limited by law).
Corporations are artificial creations of governments (in the U.S., through the Secretary of State's office in a particular state). No group can decide to give themselves limited liability, immunity from prosecution for corporate crimes, or other privileges. Only governments bestow such privilege. As government creations, corporations should be subject to democratic control, not enabled to control democracy.
The many special powers and privileges the government grants corporations (e.g. limited liability, perpetual lifespan, etc.), make limiting their political power essential.
Corporate personhood allows giant corporations to undermine free enterprise by extracting political favors that distort market competition. This harms the vast majority of businesses seeking to prosper by providing needed goods and services, rather than through legal bribery. Two of the three broad-based national business organizations filing briefs in the Citizens United case argued against expanding corporate political speech.