Voting Republican is positively correlated with being married and having children. W won 25 of the 26 states with the highest fertility. Gore and Kerry got their highest margins in the states with the lowest fertility. The DemocRATS agenda has gotten more and more hostile to traditional families. As Republican portions of the country outbreed DemocRAT portions, power will shift to the regions with more population growth.
People move though, and politics aren't genetic (ask my parents). South Dakota, for example, has a high birthrate but a low population because they lose many of their young people to Wisconsin, Minnesota, and Illinois.