We all know the subtiutle of Das Kapital. But Marx did many of his categories from the classical Political Economy, and can be considered at once putting forward the Critique of Political Economy and constituting its zenith. Smith and Ricardo had labor theories of value, distinctions between natural and market prices, .e.g., etc.
Political Economy was more than 'economics'. It included history, philosophy, and what we now consider sociology, political science, etc. And the whole was greater than the sum of its parts. The breaking up of political economy into the narrower socials sciences with which we are familiar in the late 19th century was promoted by 'neoclassical' marginalists like Jevons. Of course, Marxian and other traditions continued to stay alive, and to revive an reconstruct politcal economy, and critique.
But there is no doubt that Marx investigated the determination of value and distribution, competition and technological change, cycles and accumulation, all objects of analysis of political economy, and even to a certain extent, economics (though the change from political economy to economics resulted in a change in the object of inquiry from social reproduction and material provisioning to 'constrained decision making'--but this raises many other big issues).
Mat