The most important consequence of allowing prisoners to vote is that it would remove the incentives for “prison gerrymandering.” In most U.S. states, prisoners are counted by the census based on where they are incarcerated, not where they are registered to vote. Because most large prisons are in sparsely populated rural areas, prison complexes have an important effect on gerrymandering.
Many prisoners are racial minorities or people who live in urban areas, which means these places lose voting population, while more conservative areas gain nonvoting population. This advantages Republican congressmen in places like upstate New York, who benefit from inflated populations for redistricting purposes, but have nothing to fear at election time. Prisoner disenfranchisement therefore contributes to a structural disparity that causes Congress and state legislatures to be more conservative than the public at large.