Politics is by its nature a changeable thing, something that despite the advent of "political science" still manages to be a fickle thing, not responding to theories of unbreakable rules. This is because politics is related to particular, rather than universal things, particular politicians, particular issues, budgets, and voters, all of which are subject to the particular circumstances of a time. In other words, a political campaign which one time worked in one state might not work in another, or even the same state at a different time.
A purely political art then is a work of art that is used to promote a particular political end. A purely political art is most properly called propaganda, it is used to propagate, promote and convince people of the goodness or importance of an issue. After this particular issue or cause is no longer in play, the purely political art loses its moorings and becomes meaningless, art then becomes merely an item of curiosity. The art of the Chinese Cultural Revolution comes to mind, the posters don't really move you to anything other than finding the design striking and interesting.
Politics and politicians however are not guided simply by particular circumstances, but rather (at least the best of them) are guided by principles that are applied to particular circumstances. The principles are what a philosopher would call universals. The universal truths, such as justice, equality, courage, et cetera. When we see these universal principles as the guiding force of work of a politician, rather than simply the expedient, we acknowledge this as a great thing and label such people "statesmen" rather a politician.
Art then works the same way. The universals are at work in the best works of art, the courage of a man, the need for justice, the longing for beauty, these make the best art universally loved, thus we call it "Art." On the other hand, art which is purely used for political ends, which has little or no value in the universal sense, but is valued as we said only for the particular circumstance of the time and place, is called "propaganda." But yet, even in art which is intentionally political, which might be called propaganda in some sense, still can express the human virtues in a universal way.
Looking at Goya's The Shootings of May Third, we do not have to understand the particular political event which inspired the painting to be inspired by this. The defiance of the man with his arms stretched out stands out. He may be a radical, or a monarchist or whatever party, but his courage is what strikes everyone viewing this painting, this universal virtue turns Goya's painting from propaganda into the realm of true art.
Thus it is the universals, the philosophical, which makes art what it is, but the purely political degrades art into the realm of propaganda. The artist stands to the propagandist like the statesman
stands to the political hack.
But what happens when the propagandist uses, or more correctly misuses art for terrible ends?