It might be deemed unthinkable that the historic administration before Obama would meet this description, but what if the prior administration were dominated by dogmatic racists? What if the Secret Service were over an 8 year period gradually purged in favor of racist beliefs, and in the interim between the election and the new President-Elect taking office, the operation were planned with the outgoing President and key staff knowing all about it. (But plausible deniability holds; the higher administrative involvement is kept off the record, no paper trail, and the agents involved are willing to sacrifice themselves and keep silent--in fact, after the job is done they commit "honorable" suicide so no one can question them).
I will admit I figured it was quite possible the Bush administration might leave Obama a SS that was, if not transformed into such a monolithic spear, at any rate salted with people who might not be as reliable on the "race does not matter" assumption as posters here have so blithely assumed. It is not like the kind of racism perpetuating Justice Taney's "no rights a white man is bound to respect" attitude is unknown in the USA, I would guess that at least 1 percent of self-identified "white" people and maybe an order of magnitude higher are that extreme, at least they think they are, having not dared themselves into acting on their beliefs in an extreme manner. But the 1 percent would include a lot of people who have. Most of these would go around sporting swastika tattoos and have a criminal record (indeed prison is probably where a lot of these attitudes get baked in good and hard) but it would always be possible to find people of this sort of mentality, of the Tim McVeigh type, in places like the Army. A white supremacist prior administration would have to go out of its way to make explicit belief in racist doctrine dominant in the SS, and I suppose purging the prior cohort, which would involve flushing out at least a few individuals with a serious belief in the more liberal notions of the American creed (I have read the memoirs of such an individual who had served in the SS) would be a danger point--some former member would go to the press. But perhaps an administration that first incorporated outspoken if canny white supremacists into the Secret Service and then, observing a non-white candidate elected despite their no doubt pulling out the stops in dirty tricks to prevent that, resolve to have them assassinated, would not stick at effectively silencing such stray elements. It is all right if some alarming stories get out into the alternative press--if one is the kind of person to pay attention to the non-mainstream papers, one is aware of, or anyway believes in, some really scary stuff anyway that no one ever seems to act on to correct!
But since Secret Service is the source of the agents who are set to guard Presidential candidates as well as actually elected individuals, it would be just as easy, or just as impossible if one takes the point of view that the suppression of American overt racism is irreversible, to kill the President-elect long before inauguration; indeed long before they could actually be elected. Certainly it is not unprecedented for successful political candidates, like Robert Kennedy or George Wallace, to be shot down before they can win a nomination, let alone the actual office.
It would always be unnecessarily stupid to display the fact that the Secret Service sworn to protect the President was itself the guilty party, unless white supremacists were so close to simply taking power and enforcing their views systematically that one wonders why they allowed the black guy to run for the Presidency in the first place. If one assumes they cannot simply rule as they wish despite having managed to install a President who is overtly racist and proud of it--well, perhaps this is an era where he has to keep that white pride better hidden than they have had to in the past couple generations! Reagan and the Bushes and Trump have all been able to telegraph quite a bit of solidarity with outspoken white supremacists, albeit always keeping it ambiguous, and Democratic candidates have been accused of the same sort of double dealing. Therefore one assumes circumstances in which overt and frank expressions of intent to exclude non-whites from all power would be met by impeachment and removal. Letting the SS be seen to be complicit in killing a winning black candidate would surely have the same outcome, even if the claim the President themself had noting to do with it could be plausibly made. "The Buck Stops Here" is the standard holding the President accountable for whatever their administration's branches might do.
So, it would be necessary to make it look like some third party managed to overwhelm SS protection; probably a few SS guys would have to die in the line of duty. It might be possible to arrange so that the ones who do are all the old guard non-racist types and all the survivors are the new wave. Even this might not prevent impeachment and ultimate exposure of the complicity of the President, as well as the unreliability of the newly revised SS, but there is a chance they could brazen it out, and one way or the other, the non-white hero is dead.