Not to dissuade further discussion, but you may be interested in our FAQ section on the Changing role of Republicans and Democrats.

The rising system of mass incarceration heavily contributed to the rise of the Republican Party during this time. The US Census Bureau counts prisoners as residents of the county where they are imprisoned. Prisons largely reside in rural counties--counties which are predominately white and Republican. As mass incarceration predominately targeted urban spaces, this took a large voting bloc from urban ethnic enclaves and transferred it to Republican areas.

For example, in the year 2000, 35% of the population of Crowley County, Colorado were prisoners. In Concho County, Texas, the figure was 33%, and in Union County, Florida, it was 30%. These overwhelmingly Republican areas came to enjoy strong political clout. 98% of some states' prisons were located in Republican counties. Heather Ann Thompson in "Why Mass Incarceration Matters" argues that this distorted population count "empowered the Republican party to a degree that no single redistricting effort could have."

In many states, convicted felons are barred from voting. Targeting mainly people of color in urban spaces, mass incarceration therefore removes many people the right to vote. By 2000, 1.8 million African Americans had been barred from voting due to felon disfranchisement, removing a base that historically votes Democrat.

Research has shown that these policies of prison population count affected the outcome of seven Senate races from 1970 to 1998 where the Democratic candidate would have won rather than the Republican. These elections then prevented Democratic control of the Senate from 1986 to 2000. Christopher Uggen argues that excluding felons from voting has provided "a small but clear advantage to Republican candidates in every presidential and senatorial election from 1972 to 2000."

Sources:

Jeff Manza and Christopher Uggen, Locked Out: Felon Disenfranchisement and American Democracy

Elizabeth Hull, The Disenfranchisement of Ex-felons

Ryan King and Marc Mauer, "The Vanishing Black Electorate: Felony Disenfranchisement in Atlanta, Georgia"

Heather Ann Thompson, "Why Mass Incarceration Matters: Rethinking Crisis, Decline, and Transformation in Postwar American History"