There could be a whole host of confounding factors. One other explanation:
Where there are strong knowledge-based economies, there's a need to delay childbearing until one has the necessary education to be economically competitive. This results in low fertility rates, and it also results in high education levels. High education correlates with a liberal political orientation. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.
or:
Where there are large numbers of skilled immigrants, people tend to lean liberal, as the cultural melting pot "desensitizes" them to new ideas. This exchange of ideas also spurs a bustling idea marketplace and healthy startup economy. This, in turn, results in higher levels of income inequality, which means that people have a large incentive to play the lottery and delay childbearing until they've had their shot at the big bucks. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.
or:
Smarter people tend to be educated. Educated people tend to be liberals. Smarter people also tend not to get pregnant accidentally. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.
Liberal politics in the United States is driven largely by unionism. I don't see how the economic protectionism and populism of US liberals develops from high education and cultural melting-pots.
Where there are strong knowledge-based economies, there's a need to delay childbearing until one has the necessary education to be economically competitive. This results in low fertility rates, and it also results in high education levels. High education correlates with a liberal political orientation. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.
or:
Where there are large numbers of skilled immigrants, people tend to lean liberal, as the cultural melting pot "desensitizes" them to new ideas. This exchange of ideas also spurs a bustling idea marketplace and healthy startup economy. This, in turn, results in higher levels of income inequality, which means that people have a large incentive to play the lottery and delay childbearing until they've had their shot at the big bucks. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.
or:
Smarter people tend to be educated. Educated people tend to be liberals. Smarter people also tend not to get pregnant accidentally. Therefore, liberal political orientation correlates with delayed childbearing and lower fertility.