Do Democratic Presidencies Reduce Abortion Rates?

Of course not.

Explanation for the Political Divide?

Perhaps a partial explanation (though I don't know how scientific). 

In r/K Selection Theory in Evolutionary Ecology, if you provide a population with free resources, those who will come to dominate the population will exhibit five basic traits, called an r-selected Reproductive Strategy. These traits are all designed to best exploit the free resource availability. In nature, the r-selected strategy is best seen in the rabbit, which lives in fields of grass it will never fully consume. The five traits are, competition and risk avoidance, promiscuity, low-investment single parenting, earlier age of sexualization of young, and no loyalty to in-group. These traits are ultimately designed to selfishly maximize the numbers of offspring produced. Each of these offspring, though of lesser fitness, will be able to survive and reproduce freely themselves, due to the free resource availability. In r-selection nobody ends up dead, and killing or being killed is not a concern.

In r/K Theory, there is also a strategy exactly opposite to the rabbit's, which emerges under conditions of resource scarcity. It is called the K-selected Reproductive Strategy. There, where resources are scarce, competition for resources is everywhere, and some individuals will die due to failure in competition, and the resultant resource denial that this produces. This produces the K-strategy, which is best seen in the wolf. This strategy also has five psychological traits - competitiveness/aggressiveness/protectiveness, mate monopolization/monogamy, high-investment two-parent child-rearing, later age of sexualization of young, and high loyalty to in-group. This psychology is designed to form highly fit and competitive groups that succeed in group competition, all while capturing and monopolizing the fittest mate possible, as a means of making their offspring genetically fitter than those of competitors. Here, the goal is not to simply consume as much as possible yourself and produce as many baby-making machines as you can, with little regard to their fitness. Here, the goal is to help your group succeed in its competition for the scarce resources, and then produce offspring of as high a fitness as possible, so they may carry your genes forward by succeeding in competition themselves. It is obvious why every r-strategy ideal would act as a repellent to a K-strategist, since each ideal would guarantee failure in the K-selected environment.

The premise of this highly substantiated scientific work is that all of politics is really a battle between the K-strategist wolves within our society, designed to battle in a world of scarce resources and fierce competition, and the r-strategist rabbits, designed to freely graze the bounty of a sudden resource glut and rapidly explode in numbers to exploit such a glut.