We link future members of Congress to the de-anonymized 1940 census to offer a uniquely detailed analysis of how economically unrepresentative American politicians were in the 20th century, and why. Future members under the age of 18 in 1940 grew up in households with parents who earned more than twice as much as the population average and who were more than 6 times as likely as the general population to hold college degrees. However, compared to siblings who did not become politicians, future members of Congress between the ages of 18 and 40 in 1940 were higher-earners and more educated, indicating that socioeconomic background alone does not explain the differences between politicians and non-politicians. Examining a smaller sample of candidates that includes non-winners, we find that the candidate pool is much higher-earning and more educated than the general population. At the same time, among the candidate pool, elections advantage candidates with higher earnings ability and education. We conclude that barriers to entry likely deter a more economically representative candidate pool, but that electoral advantages for more-educated individuals with more private-sector success also play an important role.