An older, now dying, generation of conservative jurists based their opposition to Roe on a positivist argument: The Constitution says nothing about abortion (or a right to marital privacy, which was established in the Griswold decision of 1965, and on which the right to an abortion was based in Roe). Therefore there is no possible constitutional right to abortion. Which means that, like any non-constitutional issue, abortion should be left in the political arena, with states deciding whether or not to allow it, and whether and how to regulate it.
But for an influential faction of pro-life intellectuals, that position is morally unacceptable — and for good reason. If an abortion really is an extrajudicial act of lethal violence against an innocent human being, then allowing democratically elected legislatures to decide whether or not to treat this act as a crime seems to be monstrous. Murder is murder, whether or not the voters of a state recognize it as such. To allow the legislatures of more liberal states in a post-Roe order to keep abortion legal would therefore be, in the words of influential abortion opponent Robert P. George, a betrayal of the “substantive principle of equal worth and dignity that is the moral linchpin of democracy.” For that reason, the fully pro-life position requires the unborn to be defined as persons covered by the equal protection clause of the 14th Amendment.