This post starts with three main observations.
One is that Social Darwinism, whether under that name or not, has a long history in America.
The second is that, in general, this has been more a respected than a disrespected position in American social, political and even religious thought.
The third is that, along with increasing income inequality, a renewed focus on Social Darwinism started in Reagan's time.
That said, let's jump to the theme of the header.
I see modern American Social Darwinism falling into four main streams.
One of them is definitely rooted in its earliest days. The second, under somewhat different guises, is, too. The third goes back a century or so, longer than most people think, and has permutated since then. And the fourth is brand new, and will probably shock some people to be listed here.
More on all four, how and why each qualifies as Social Darwinistic, etc., below the fold.
The first is hypercapitalism/economic libertarianism. No duh, right? This one's been around since before the phrase "Social Darwinism" was even coined. Arguably, given the fairly conservative nature of the American Revolution, and the even more conservative, and propertied, Constitutional Convention, this version has a bit of root in America's founding.
The second is known today as the "success Gospel" or "prosperity Gospel." However, it's arguable this goes back to John Winthrop's "city on a hill," and the whole idea among dissenting, non-Anglican Protestants, that god was blessing them, or would bless them, for their righteousness. (Often, at either the expense of dispossessed American Indians or enslaved Africans.)
The first and second have often intertwined, in either sincere embrace, or hypocritical fornication. That's true today, and it was true back in the Gilded Age.
The third? It's the New Age's "name it and claim it" belief system. That said, this began as "New Thought" Christianity, what lies behind today's Christian Science, various New Thought movements of the 1920s-30s that led to today's Unity School of Christianity and more, long before the New Age, with Aquarius and hippies, put more of an Eastern-religion patina on what was already Eastern-influenced heterodox Christianity.
Pre-Aquarius days, New Thought Christianity influenced the 12-step self-help movement, though it picked up steam after that, in the 1970s.
This is not necessarily materialistic, but can be focused on the idea one can will oneself to better help, to a new lover, etc. At the same time, some new thought within fundamentalist Christianity is picking up on that.
The fourth? Today's Gnu/New Atheism. Now, Gnu Atheism isn't explicitly money-driven. But, it is quasi-libertarian individualistic in many of its manifestations and it IS Social Darwinistic in things like psychology, claiming with Nietzsche that religion is for the "weak," etc.
Obviously, hypercapitalism, or any capitalistic economics that cannot work for the betterment of larger groups, and that claims that money is a measure of "progress," must be rejected. No man, no matter how rich, is an island.
So, too, the success gospel must be rejected. To say it's untrue to the Judeo-Christian scriptures is not totally true. Places in the Old Testament/Tanakh drip with success theology. That's part of why it has to be rejected. A god who encourages people to measure their divine anointing by mammon is a paltry god indeed.
The materialistic side of New Ageism must be rejected for similar reasons. But, even more, the non-monetary side of New Age Social Darwinism must be vehemently rejected. The idea that one's cancer has recurred because of one's mental weakness, whether through inadequate trust in crystals or inadequate prayers to Jesus, is horrible.
To many freethinking minds, none of this is new.
But, Gnu Atheism's version of Social Darwinism must also be rejected. It has a very simplistic view of the origins of religious belief both in Homo sapiens as a species and in individuals in our species. It flirts with, or crosses over into, scientism. It's generally not humanistic. And, it's bad PR for atheism and agnosticism. And, for the atheist who admits some such "longings," like the AA member who admits, even with years sober, an occasional "longing," it's a way to ostracize people, too.
And, that's probably true to some degree of all the varieties of Social Darwinism; doubt its verities within your group and become subject to ostracism.