"Darwinism" is generally used as a stand-in for "evolution." But there's a reason that it is referred to as "Darwinian evolution;" it is "evolution via natural selection." But when people attack "social darwinism," for example, they're attacking some Hitleresque utopian ideal where we artificially select who will live and who will die based on some genetic criteria or other. They're not attacking the idea that such a process would actually cause a change in the allele frequency of the human population at large (even though that actually would constitute an attack on evolution, which is the idea that the allele frequency of a population changes over time due to a combination of genetic drift and natural or artificial selection) --- which is to say, they're not attacking the idea that selection actually works to cause change in genetic frequency. Rather, they're attacking the idea that we should artificially select for such change, and specifically, against the idea that we should do so at the cost of human life or happiness. The idea is that even if natural selection does cause a change in genetic frequency, then it's still wrong to select for such change artificially in a way that forcibly hinders people's lives or rights. Which is of course true, even though natural selection does happen, and evolution does occur (and would occur in some Hitleresque utopian society where only the blonde-haired and blue-eyes were allowed to breed).
So keep this in mind: when someone attacks "Darwinism," or "social Darwinism," a couple of things are likely to be true:
1) They think they're attacking the credibility of evolution (they're not);
2) They think they're attacking the credibility of natural selection (they're not);
3) They think they're attacking the morality of artificial selection among humans (they are, and rightly so).
The important thing to remember here is that the morality of artificially enforcing such a policy in a human society has *nothing whatsoever* to do with the evidence that such a process would actually do what it set out to do; whether evolution and natural selection happen or do not happen, this has no effect on the moral aspect of doing so artificially, because the moral argument is against hindering people's lives or rights, regardless of the reason.
I can sum up the entire point of this post as follows:
1) Hitlerian artificial selection is morally reprehensible for reasons that have NOTHING WHATSOEVER to do with whether or not such selection would actually produce genetic change. It has to do with the value placed on human life, which can be argued regardless of the truth or falsehood of evolution.
2) To put it in context; saying that evolution doesn't happen because enforced human artificial selection is wrong is like saying, "shotguns aren't real and don't actually work because shooting innocent people in the face is bad."
If you still don't understand (or would just like further, more elegant extrapolation on the subject), I would recommend this segment of an interview with Richard Dawkins, from Penn Jillette's radio show, in which Dawkins explains this relationship in a straightforward and interesting fashion (starting at about the 3:00 mark):
"That whole thing became very, very unfashionable, I think, partly because of Hitler. Because Hitler, of course, was the arch-eugenecist, and Hitler was completely, as we know, screwy...I mean, Hitler has given all kinds of eugenics a bad name. I agree with you, I think it would be a very unpleasant thing if there were to be a sort of government-organized breeding program, to say, 'let's encourage the smart people to breed and encourage the dull people not to,' I think that would be a horrible thing. That's different, however, from saying that it wouldn't work. And I hear a lot of people who just go around saying, 'it just wouldn't work,' and I fear that they're just saying that because, for ideological reasons, they don't want it to work."
And that is exactly what is going on here, when people attack the credibility of "Darwinism" for moral reasons.