Political orientation has been shown to correlate with fertility, raising the possibility that demographic processes contribute to long-term ideological change. Using data from the US General Social Survey, we analyze completed fertility across 17 birth cohorts (1898–1982) to examine how political orientation has contributed to fertility decline in the United States and whether emerging selective forces can be detected. Earlier cohorts show little difference in fertility by political orientation. From the 1943–1947 birth cohort onward, however, a pronounced divergence emerges: individuals with right-wing political orientations maintain fertility at or above replacement level, whereas fertility among left-wing individuals declines sharply to well below replacement. Applying Lande–Arnold selection gradient analyses, we find increasing directional selection that may favor right-wing political orientation over time, while education shows consistent negative associations with fertility and religiosity positive but weaker effects. Separate analysis of Black and White Americans reveals, however, that the increasingly stronger association between political orientation and fertility in more recent cohorts holds only true for whites but not for blacks. Nonetheless, these findings suggest that recent fertility decline in the United States is driven disproportionately by left-leaning individuals and point to contemporary demographic processes that may gradually shift the ideological composition of populations.