IMO the stand US Republican and Democrats take on various social issues is completely illogical from the viewpoint of political philosophy. I cannot decipher the true underlying philosophy behind the choices so-called "left" and "right" make.
For example, I tried to figure out how the parties measure up on the principle of liberty. On the gun control issue Republicans insist that people have right to bear arms; not the Democrats. On the other hand, on the gay rights issue Democrats side with liberty, not the Republicans.
Something similar happens on the principle of the sanctity of human life: on the issue of abortion Republicans favour life, but on the issue of death penalty the Democrats do.
The same happens for every underlying principle I could think of, liberty, safety, etc: the relative stand of the two major party on various social issues is not consistent according to any given principle.
Therefore the question: what underlying principles, if any, determine the major US political parties' stands on issues?
I've got to explain why "voter base" explanation doesn't resolve the question.
Suppose political parties D and R address issues Q1..Qn and sided with answers D1..Dn and R1..Rn respectively. Suppose that each party attempts to maximize their voter base. What would that say us about correlations Corr(Di,Dj) and Corr(Ri,Rj)?
Since each party attempts to strengthen their voter base they wouldn't pick combinations of issue position that would alienate the voters. Therefore Corr(Di,Dj)>0 (as well as Corr(Ri,Rj)>0) implies that there is a positive correlation between voter positions on issues Qi and Qj.
Therefore on average the voters who agree on one issue with their favourite party would also agree on another issue with the same party. If that wasn't so the parties would choose a different issue division among them in order to avoid alienation of their voter base.
Hence there are some underlying causes, perhaps psychological one among voters, for the particular division of the issue stands among the parties. Therefore an answer "parties choose their stands on the issues to attract more voters" to a question "what are the reasons parties correlate their stand on issues" begs another question: "what are the reasons voters correlate on average their stand on issues?"
For example, there is no question that Republican party side with liberty on the gun issue and against liberty on the gay rights issue. It is also very much possible that these choices are made to attract their voter base. However, this begs the question why most people who vote Republican side with liberty on the gun issue and against liberty on the gay right issue. Moreover, there is plenty of evidence that this is indeed so: the majority of Republican votes indeed have the described above opinions.
Which brings us back to essentially the same question: what are the cultural, psychological, or philosophical reasons for the seemingly inconsistent choices the voters have on the variety of issues?