In 1868 the 14th Amendment to the United States Constitution had become law. Section 1 of that Amendment states:
SECTION 1. All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
"The one pervading purpose . . . [of the 14th Amendment] was the freedom of the slave race, the security and firm establishment of that freedom, and the protection of the newly-made freeman and citizen from the oppression of those who had formerly exercised unlimited dominion over him." That is exactly what Justice Samuel F. Miller said in 1873 in one of the first Supreme Court opinions to rule on the 14th Amendment. [83 U.S. 36, 81 (1873)]
But the wealthy, powerful men who owned corporations wanted more power for their corporations. Their lawyers came up with the idea that corporations, which might be said to be groups of persons (though one person might in turn belong to (own stock in) many corporations), should have the same constitutional rights as persons themselves. If they could get the courts to agree that corporations were persons, they could assert that the States, which had chartered the corporations, would then be constrained by the 14th Amendment from exercising power over the corporations.
Beginning in the 1870's corporate lawyers began asserting that corporations were persons with many of the rights of natural persons. It should be understood that the term "artificial person" was already in long use, with no mistake that corporations were claiming to have the rights of natural persons. "Artificial person" was used because there were certain resemblances, in law, between a natural person and corporations. Both could be parties in a lawsuit; both could be taxed; both could be constrained by law. In fact the corporations had been called artificial persons by courts in England as early as the 16th century because lawyers for the corporations had asserted they could not be convicted under the English laws of the time because the laws were worded "No person shall..."
The need to be freed from legislative and judicial constraints, combined with the use of the word "person" in the U.S. Constitution and the concept of the "artificial person," led to the argument that these "artificial persons" were "persons" with an inconsequential "artificial" adjective appended. If it could be made so, if the courts would accept that corporations were among the "persons" talked about by the U.S. Constitution, then the corporations would gain considerably more leverage against legal restraint.
These arguments were made by corporate lawyers at the State level, in court after court, and many judges, being former corporate attorneys and usually at least moderately wealthy themselves, were sympathetic to any argument that would strengthen corporations. There was a national campaign to get the legal establishment to accept that corporations were persons. This cumulated in the Santa Clara decision of 1886, which has been used as the precedent for all rulings about corporate personhood since then.