Conservatism and evangelical Christianity have a tight relationship with the Republican party. At first glance an outsider might think that evangelicals associate themselves with the Republican party out of some historic family loyalty or tradition. It looks like evangelicals are owned by the Republican party. At least that’s what pop news tells us.
But how did this relationship come about? For that we go back about two centuries to the birth of conservatism. Or rather what we call conservatism. You see, conservatism is really the first progressive movement. It served as the foundation for America’s sense of liberty and justice. Conservatism is the great experiment in self-government and modern democracy.
This man named Edmund Burke was the one who encapsulated all of the ideas floating around at the time. He rejected the French approach, that of the Rationalist and the Jacobin, with respect to human freedom. Rationalism represented to him a radically libertarian mind and life. These things would never suffice because they were wholly inadequate. A society requires a foundation other than the perceptions of the individual mind. It needs something outside of itself, a higher standard.
But that was in England. How did this influence America? Before the American Revolution he engaged the colonists and Parliament in finding solutions to problems such as the Stamp Act. Reconciliation proved difficult if not impossible and always short-lived. Yet his ideas persisted and were adopted. For instance
Burke rejected the notion that a representative was merely the hired delegate of his constituents, bound to vote as directed. Instead he asserted that representatives like him were elected to provide judgment as well as mere votes, and to apply their judgment as they deemed best calculated to further the interests of those who put him in office, as well as the nation as a whole. These two visions of representation would come to dominate the Federalist and Anti-federalist visions of government in America. The partial victory of the more independent theory of representation was crucial to the Federalist victory embodied in early republican constitutionalism.
This is one reason why we are not a simple direct democracy as people still propose. We are a nation of both representatives and judgements. In this sense he was asking that the British (and later the Americans) practice the Christian ethic in law and governance. This is consistent outgrowth of his rejection of Rationalism with its internalistic notions.
That’s ok for governing a people in general, but what about America’s original sin, slavery? Wasn’t Burke soft on that issue?
A quick google search on the subject “burke and slavery” reveals something odd. One will find that he is harshly criticized for not taking an outright abolitionist position. At the same time he did take a position. His gradualist approach was against the slave trade. One might liken it to the anti-slavery of the north as it attempted to encapsulate or contain the problem to the southern states.
We know now that containment is ineffective. It did not work with slavery. It did not work for Neville Chamberlain. It did not work as a cold war strategy. It is not working with ISIS & other jihadists. Containment does not work.
Yet the suggestion that not being an outright abolitionist is soft misses the point of being anti-slavery in principle and practice.
A more contemporary parallel might be the pro-life movement. On one side is the abolitionist movement. This movement makes plain to the general public the horror of abortion but unfortunately misses the other pro-life concerns of infanticide and euthanasia. It is rigid and critical of those who take the alternative position.
The other position in being pro-life is to be incremental. We can, it is held, take a step-by-step approach in education, in law, and in elections to whittle away at the problem. This approach has shown itself to be better at addressing the broader concerns but is still slow to accomplish its goals. Incrementalism is a difficult process.
Like the slavery question the incrementalist pro-lifer has sometimes made unnecessary compromises. Sometimes laws are drafted which would appear to secure abortion during the early months of pregnancy. That’s difficult language to undo. Like the Missouri compromise it would seem to make the problem worse by making giving it permanence.
The point is that Burke was anti-slavery. He may not have been the hard-liner that critics can use as a scapegoat or straw man but he was anti-slavery. The end of slavery in the US was influenced by the Christian ethic. It was not a rationalist matter. It was not a religion-free rationalist or scientific matter.
As the anomaly of a theology of slavery in the south  began to fade into oblivion the more evangelical of the north naturally migrated away from the party of slavery.
Then came the Progressive movement. This cemented the association. Progressivism represented (represents) a soft variety of Marxist thought. It is highly idealistic, highly impractical for the conservative. But the division was not always clear or immediate. The progressive movement instituted a number of social changes , many of which involved the evangelical voice.
This happened because of theology. In the 19th century the theology of the evangelical sought to establish God’s kingdom by establishing justice and peace. The progressive borrowed these principles  but mixed them with the principles of Rationalism. Thus the Christian brought an anti-abortion message into the suffrage movement while at the same time Sanger brought the Marxist/Malthusian approach to birth control questions.
In this example the conservative separation becomes clear. The approach of Sanger involves an appeal to individualism and reason apart from any higher principles that might bear weight on the issue. Though the target of the Marxist is always otherwise, the language of the movement is to free the individual. It is this language which separates the conservative from the modern liberal and progressive. This is a major reason why the evangelical rejects the principles of the Democratic party. It’s not about the party: It’s about the principles they hold to. Individualism is no foundation for a stable and self-governing society.
This could be no clearer than in America’s founding. The governance provided by the Articles of Confederation represented something closer to the radical individualism of the Rationalist than the conservative desires. But it’s not simply a strong central government that the conservative asks for. Rather it is a government which is unified and informed theologically, as a cohesive nation. The Articles could never do this. The neutrality of the Constitution requires it.
 “He seems to have been captivated by the wondrous order-within-complexity generated by this suprarational social process and wished to defend it against that rationalistic mentality which refuses to comprehend the significance of tradition and custom. – See more at: https://home.isi.org/edmund-burke-old-whig
 The French legislative assembly had proceeded, not on the principles by which mature, deliberate statesmen are guided, but on an allegiance to abstract concepts of equality and the rights of man. No doctrine, declared Burke, could be more insidious to the preservation of a nation or the protection of human liberty. “Those,” he declared, “who attempt to level, never equalize” http://www.theimaginativeconservative.org/2017/02/edmund-burke-american-nation-timeless-robert-heineman.html
 Noll, Mark, “The Civil War as a Theological Crisis,” University of North Carolina Press, 2006. Dr. Noll provides evidence that the slavery theology of the American south was an anomaly both in the U.S. and globally, among all sects. It was a theological aberration, a heresy.
 Gray, John, “Black Mass, Apocalyptic Religion and the Death of Utopia,” Farrar, Straus and Giroux, 2008. Gray provides the association between the ideals of liberalism and the theology of the era.