Your use of the term "social Darwinism" is quite unclear and contradictory because Sanger and her supporters were not allowing Darwin's 'survival of the fittest' idea to play itself out. Instead they were interfering in the natural process.
Where is "the rhetoric used by some of today's extreme right wingers" regarding Eugenics, birth control, abortion , etc. If you take a look at the "pro life" movement I doubt you'll find many who support Eugenics, or abortion. It is not the Left who has traditionally been against any of this; quite the opposite in fact.
Eugenics was basically seen as a sciencey way to speed up natural selection, coupled with white supremacist beliefs. Whites were thought to be superior, but not breeding fast enough. Today's white supremacists are right-wing extremists.
"The concept of the feeble-minded menace provided a way to make the rural families, who were neither institutionalized, foreign, nor "colored," into people who were "different" from the eugenicists. Underlying the family studies and the myth of the feeble-minded menace was the theory of Social Darwinism, which assumed the existence of a struggle between the individual and society, and of an adversarial relationship between the fit and unfit classes. Eugenical family studies and social Darwinism both involved a transmutation of nature into biology and the eugenics movement frequently acknowledged its debt to Social Darwinism.
The deeply conservative implications of such philosophies included the rejection of government welfare programs or protective legislation on the grounds that such reforms as poorhouses, orphanages, bread lines, and eight-hour days enabled the unfit to survive and weakened society as a whole. From the beginning, the eugenics movement accepted the regressive implications of Social Darwinism. Karl Pearson believed that "such measures as the minimum wage, the eight-hour day, free medical advice, and reductions in infant mortality encouraged an increase in unemployables, degenerates, and physical and mental weaklings."
Pearson's friend, Havelock Ellis, known as a sex radical and free thinker, shared Pearson's elitist views, writing in his 1911 eugenicist book, The Problem of Race Regeneration, "These classes, with their tendency to weak-mindedness, their inborn laziness, lack of vitality, and unfitness for organized activity, contain the people who complain they are starving for want of work, though they will never perform any work that is given them." Ellis suggested in the same book that all public relief be denied to second generation paupers unless they "voluntarily consented" to be surgically sterilized.
One American eugenicist said harshly:
"The so-called charitable people who give to begging children and women with baskets have a vast sin to answer for. It is from them that this pauper element gets its consent to exist. . . .So-called charity joins public relief in producing stillborn children, raising prostitutes, and educating criminals."
PublicEye.org - The Website of Political Research Associates
Hmmm, sound familiar?