Titled: The Fertility Gap
Simply put, liberals have a big baby problem: They're not having enough of them, they haven't for a long time, and their pool of potential new voters is suffering as a result. According to the 2004 General Social Survey, if you picked 100 unrelated politically liberal adults at random, you would find that they had, between them, 147 children. If you picked 100 conservatives, you would find 208 kids. That's a "fertility gap" of 41%. Given that about 80% of people with an identifiable party preference grow up to vote the same way as their parents, this gap translates into lots more little Republicans than little Democrats to vote in future elections.
I'm not sure it's fertility either - I would think that Dems and Repubs are about equal in ability to get pregnant and have children - abortion and choice to remain childless seem to be a liberal trait.
Of course the Dems have an opinion on that:
As one liberal columnist in a major paper graphically put it, "Maybe the scales are tipping to the neoconservative, homogenous right in our culture simply because they tend not to give much of a damn for the ramifications of wanton breeding and environmental destruction and pious sanctimony, whereas those on the left actually seem to give a whit for the health of the planet and the dire effects of overpopulation."
Maybe the conservatives just put a greater value in the sanctity of life, the value of families and realise that bringing up intelligent, happy kids is the only hope for the future of mankind?
Reasons aside, this will have a huge effect on future voting patterns:
A state that was split 50-50 between left and right in 2004 will tilt right by 2012, 54% to 46%. By 2020, it will be certifiably right-wing, 59% to 41%. A state that is currently 55-45 in favor of liberals (like California) will be 54-46 in favor of conservatives by 2020--and all for no other reason than babies.
Kising babies may be a little easier for one party in the future...
James Taranto of Opinion Journal even has a name for this - The Roe Effect - a good read from 2004 that looks at this effect in depth.