I think the main problem is America is still a Christain country and due to that, the main America public doesn\'t like to think ahead and approve subjects such as abortion, stem cell research, gay marriage and so on. As long as this country stays mainly Christain and the goverment goes hand in hand with religion, I think we will see the Republicans being the most popular party.
However, with the current generation growing up and being less religious than ever, here in say twenty years this could all change. There is room for a big change, a change that America needs in my opinion.