Calling a racist a racist need not be strategic, only informative.
Clinton supporters calling Sanders/Obama supporters sexist: that was pure political strategy, as it had little basis in fact and brought up only ancillary issues. But Trump is an actual racist and many of his supporters, who have an incredibly obtuse view of racism as beginning and ending at klansmanship, do not see that.
lol so you are calling everyone on the other side as klansmen now.
But regardless, drilling the 'He's a racist' narrative is pretty obviously a political strategy. It's one of the most basic tactics being deployed in the political arena to call people racist if they don't like their politics, and people are easily manipulated through this mechanism. This phenomenon is very well understood and used in political strategy (seeing it is largely about swaying people one way or another).
The word has been conditioned into the population so that people respond in pavlovian fashion to it. Most people probably don't even know what it means (and who can blame them since there are a hundred definitions that people use)
So sure, just choose one of the numerous definitions available at any given moment, throw it at someone, and try to induce the pavlovian response.
But yeah, 'sexist' is used in very much the same manner quite often.
That is not to say that there is no racism or sexism in the world, or that people can be legitimately like that, but it's so useful politically that it gets over played. Boy who cried wolf.
As to Trump, who knows. Maybe he's a secret racist. I wouldn't know. The strategies remain the same regardless.