Okay, let's take this seriously. I'll even take it seriously on their terms, and rather than give the whole history of the term and every example in which it's been cited, I'll do what the Cato Institute just did on their blog, and refer to Encyclopedia Britannica. As they quoted it it:
"According to the theory, which was popular in the late 19th and early 20th centuries, the weak were diminished and their cultures delimited, while the strong grew in power and in cultural influence over the weak….The poor were the “unfit” and should not be aided; in the struggle for existence, wealth was a sign of success. At the societal level, social Darwinism was used as a philosophical rationalization for imperialist, colonialist, and racist policies..."
OK, you tell me: how is this an unfair comparison to the effect of the Ryan budget proposal that Mitt Romney is so thoroughly wedded to? It is, in fact, unambiguously Republican dogma, one of the few things that the whole party, that all factions of the party, agree upon, that the wealthy got there through superior virtue of some kind and deserve to keep all of their wealth. It is, in fact, unambiguously Republican dogma that people demonstrate their superior virtue and their right to that kind of wealth through all-against-all competition in which the losers are to be economically, if not literally, destroyed. And it is widespread Republican doctrine that if the policies of supporting winners over losers result in any imperialist conquest of other nations like it did in Iraq, or colonialist support of local dictators like it does in Nigeria, or racist policies like the mortgage industry's recent wholesale discrimination against black borrowers or the common police policy of only searching black male drivers for drugs has on black prison (and thus employment) rates, well, if what you want to do about that would in any way inconvenience society's winners, then that's unacceptable.
And those are the policies of the Ryan budget. It's short on details, but the only way to make the department-by-department, branch-by-branch numbers in it work is to further impoverish everybody who's currently impoverished, in order to preserve the most important prerogatives of Republican governance. In the Ryan budget proposals, there are only two legitimate government purposes that are so important that they cannot be cut. Billionaires in general and hedge fund and equity investors in particular don't give up any of their federal largesse, and in fact get more. And defense contractors and the large standing military, the things that give us the power to dictate terms to weaker countries, must be preserved and expanded. Literally everything else, from disease prevention to law enforcement to education to retiree pensions and healthcare, must be slashed to zero, if that's what it takes to protect the prerogatives of the powerful, the strong, the wealthy.
Now you tell me: whether they call themselves that or not, how is it unfair to call that Social Darwinism?