I think Joe Carter is a little off to call this a single issue—it’s bigger than that—but aside from that caveat, he nails it:
The single issue that determines my vote—and I believe should determine how all Christians vote—is justice.
As Hunter Baker has explained, justice results from enforcing a moral order, which protects the freedom of human beings from malignant interference. A key component of this moral order is the recognition of human dignity as the foundational principle of freedom and human flourishing. Although the terms are not interchangeable, I believe the term “sanctity of life,” as defined by David Gushee, could serve as the standard definition for human dignity:
The concept of the sanctity of life is the belief that all human beings, at any and every stage of life, in any and every state of consciousness or self-awareness, of any and every race, color, ethnicity, level of intelligence, religion, language, gender, character, behavior, physical ability/disability, potential, class, social status, etc., of any and every particular quality of relationship to the viewing subject, are to be perceived as persons of equal and immeasurable worth and of inviolable dignity and therefore must be treated in a manner commensurate with this moral status.
Photo © 2012 by Wikimedia user Shyamal. License: Creative Commons Attribution-Share Alike 3.0 Unported.