Journalist Steve Sailer made the important point that white fertility was substantially higher in Red States than Blue States, something he calls "The Baby Gap". I would like to update and expand on his insight.
First, here is the correlation between total fertility per women in 2008 and McCain's vote share in 2008. The association is strong. In the latest available figure the total fertility rate of non-Hispanic white women is 1.86. You need 2.05 for replacement levels. The total fertility rate of Utah is 2.7, compared to 1.7 in Vermont.
On occasion patterns that are true in the aggregate are not true at the individual level. For example, rich states vote much more Democrat, but voting for President Obama in 2008 was actually negatively associated with income. The reason for the paradox is the *within* Red states in particular and to some extent also within blue state, income is associated with voting Republican.
For this reason I have evaluated the hypothesis using micro-level data. The source is National Annenberg Election Survey, which is essentially a super-poll of tens of thousands of Americans, with lots of questions.
I am looking at women 40-45 to get a good proxy for lifetime fertility. At this age mostly the children have not left home yet, and above this age the probability of having more children is extremely low.
Overall, conservative non-hispanic white women have 26% more children than liberal white women, a sizable difference.
The very conservative have 45% more kids on average than the very liberal.
So what does this imply? Political preferences are strongly related with parental views. This is probably not genetic. The likelihood to vote has been shown to be to some extent genetic, but not who you vote for. The link is cultural. Still how the ideology is transmitted is not important, just that it is.
Let us assume that if you have conservative parents, you become conservative with 70% probability and liberal with 30%, and vice-versa. With this number if you start off equally divided between liberal and conservative (there are more of the later, but many moderates are really liberal who don't want to admit it) in 1 generation the right will be 10% larger than the left.
Of course at the same time because of immigration and very high Hispanic (especially Mexican) birth rates, the Hispanic share is rising. Hispanics and African American voters are far to the left of middle, especially on fiscal issues.
The higher Conservative birth rate slows the demographic ascent of the current, liberal leaning Democrat party. However liberal white voters, who are ideologically very committed, will be replaced by equally left leaning but less ideological minority voters.
Based on these 2 long term demographic trends whites will become increasingly republican and the Democrat party increasingly minority.
Of course these long term trends are dominated by short term events. In 2004 Kerry won the overall young vote (18-29) by 9 points, while Bush won the white young vote by 11 points. In 2008 President Obama won the young vote by 34 points, and the white young vote by 10 points (54-44%).