As a classical liberal in the vein of J. S. Mill and John Rawls I am also upset by the misuse of the word these days.
I would add, however, that classical political liberalism does tend to treat economic issues *somewhat* differently from other issues, whereby things like progressive income taxes and robust social safety nets are not at odds with the traditional liberal and egalitarian political philosophies dating back centuries. Thus, one of the main ‘in practice’ liberal/libertarian distinctions.
But, indeed, current usage of “liberal” as a synonym for “left” is exactly as rediculous as thinking Mill and Marx are interchangeable.
What I've found though, is that "Trump supporter" and "dumb" are quite succinct in their descriptions of each other.