The roots of corporate 'personhood'
In Economic Power and the Corruption of the American Political System, Jeremy Cloward of Diablo Valley College traces the roots of corporate influence in the U.S. political system to the Supreme Court's decision in Dartmouth College v. Woodward, which recognized a corporation's right to have its charter honored as a contract. By extension, the decision granted individual property rights to corporations. Then in Santa Clara County v. Southern Pacific Railroad, the court ruled that the Fourteenth Amendment applied to corporations as well as to people. While Cloward and other observers chalk up this finding to a clerical error, subsequent Supreme Court decisions cemented the legal standing of corporations as people.
What's missing from decisions stretching from Pembina Consolidated Silver Mining Co. v. Pennsylvania (1888) to Citizens United is a clear explanation of the court's rationale for its determination. In some instances when finding liability or criminality, courts distinguish a corporation from its officers/agents (and shareholders), while in others a corporation and its officers, managers, and shareholders appear to be synonymous. Consider who is actually doing the "speaking" when a corporation exercises its Constitutional right to free speech.
 Dartmouth College v. Woodworth (1819) 17 U.S. 518.
 Santa Clara County v. Southern P. R. Co. (1886) 118 U.S. 394.
 Pembina Consol. Silver Mining & Milling Co. v. Pennsylvania (1888) 125 U.S. 181.