A common argument used by conservative Christians against evolutionary theory is the idea that it reduces humanity to mere physical beings and morality to "survival of the fittest." They don't often mention Social Darwinism by name, yet these same conservative Christians often support politics similar Social Darwinism in their effects, if not their intent. How can Christians be so horrified by the alleged moral and social consequences of "Darwinism" while supporting Social Darwinism?
Evolutionary theory describes how species adapt and change in the context of constant struggle and competition. Social Darwinists seek to apply something like this to the structure and nature of society, arguing that those who "fail" in competition with others for resources should just be left to their fate so that the "winners" can move on. There's too much wrong with Social Darwinism to detail here — not just morally, but also in it's understanding and application of evolutionary theory itself. Charles Darwin himself was not a Social Darwinist and there is nothing about evolutionary theory which requires or even strongly suggests that Social Darwinism might be a good idea.
What matters most is the fact that even if they don't mention it by name, such negative moral and social consequences are an important part of conservative Christians' objections to teaching evolutionary theory. If Social Darwinism were a necessary consequence of teaching evolutionary theory, they'd have a point — though even in such a situation, that wouldn't be evidence that evolutionary theory is wrong. Should we really refrain from teaching the truth if the truth leads to unpleasant consequences?
Aside from that, if conservative Christians really are sincere in their objections to Social Darwinism, why are they so vociferous in their defense of economic and political policies which have similar effects: the poor keep being left behind while the rich amass ever more power. True opponents of Social Darwinism should be strong supporters of social safety nets and welfare policies which ensure that everyone can have a minimally decent standard of living, basic health care, good education, etc. In short, strong opponents to Social Darwinism should tend to prefer the policies of liberal Democrats over conservative Republicans.