In my opinion, it is more pertinent to speak of the social issues that Parks is addressing than the political ones. Therefore, although it is interesting (and even ironic) that "history repeats itself" in the idea of Booth shooting Lincoln once again, and the presidency always has to do with politics, your question can best be answered by focusing on two social issues: the despair of poverty and the frustration of the impoverished.
Both Lincoln (topdog) and Booth (underdog), as brothers, are burdened with poverty. This social problem of poverty produces a feeling of despair in the two brothers. The feeling of despair in this poverty leads the two characters to do different things. The topdog, Lincoln, takes the "right" road by always endeavoring to get a legitimate job, first as an impersonator of President Lincoln and second as a security guard.
People know the real deal. When people know the real deal it ain't a hustle.
Even his being fired from the first job (due to the cost-saving use of a dummy instead of a real person) doesn't deter Lincoln enough to enter the world of hustling full-time; he simply gets another legitimate job as a security guard. Booth's despair leads him down a darker path: the path of a hustler. Booth is always trying to master Three Card Monte in order to obtain money in a more devious manner off the streets. He eventually begins to shoplift and finally becomes insistent in playing the card game for the brothers' only inheritance.
The social problem of poverty also underscores frustration. Although both characters are frustrated by their poverty, it is Booth who takes the frustration to a negative extreme until it becomes actual violence. Lincoln's frustration should be noted in the scene where he actually gets drunk in his Lincoln costume. The rest of Lincoln's decisions, however, come from despair (and acceptance of his situation) and not frustration. It is Booth who is most burdened by this frustration which leads to most of his poor decisions. His frustration leads him away from the legitimate means of obtaining money (a job) and draws him towards the life of a hustler (card player).
I told her I was the little brother and the big brother should look out after the little brother. She just said it again. That I should look out for you. Yeah. So who gonna look out for me. Not like you care. Here I am interested in an economic opportunity, willing to work hard, willing to take risks and all you can say you ***** eating ************* pathetic limp**** uncle tom, all you can tell me is how you don't do no more what I be wanting to do. Here I am trying to earn a living and you standing in my way. YOU STANDING IN MY WAY, LINK!
The irony is that Lincoln is good at it while Booth is not; therefore, Booth needs Lincoln to succeed. Later in the play, Booth's frustration leads him away from the gambling card game (which is still within the law) to shoplifting. Now Booth has become a criminal in his frustration with his poverty. From here Booth spirals downwards from squandering his mother's inheritance to finally committing the ultimate criminal act: violence. Booth murders his own brother.
What I find interesting is that racial issues do not make it to the forefront of this play. Some scholars go out on a limb and disagree a bit by saying the following:
By linking the two brothers to broader historical issues and personages, Parks suggests that their struggle for survival resembles the historical struggle of all African Americans in the face of racial and social inequities.
However, I find this to be reading between the lines a bit too much. Parks may "suggest" it; however, I don't think that's enough to create a "political issue" just for the sake of an essay. I am more impressed that Parks focuses fully on the aspect of poverty in itself and does a good job of separating that poverty from racial issues.