Today, the Supreme Court is hearing oral arguments in a voting rights case that has the potential to completely change the way states draw district lines and provides a big boost to Republican voters around the country. The case, Evenwel v. Abbott, concerns the "one person, one vote" doctrine that dates back to the 1960s, when the Supreme Court held that states must draw legislative districts so they contain equal populations, according to CNN.

But the court never explicitly ruled on who exactly should be counted — all U.S. citizens who live in a district or just eligible voters?

Now, the Supreme Court is hearing a case from two Texas voters, backed by conservative group the Project on Fair Representation, who want Texas to only count citizens who are eligible to vote when determining state Senate lines, rather than counting the entire population, according to McClatchy.

The plaintiffs, Sue Evenwel and Edward Pfenninger, say their rural Senate districts have far more registered voters than more urban areas, and argue that their vote is being diluted in relation to voters in other districts.

Evenwel and Pfenninger said in their court papers that they live in "districts among the most overpopulated with eligible voters," which means "there are voters or potential voters in Texas whose Senate votes are worth approximately one and one-half times that of appellants," according to the New York Times.

"From the beginning, the fundamental purpose of the one-person, one-vote principle has been to ensure that the states apportion districts in a way that protects the right of eligible voters to an equal vote," argued Evenwel's lawyers in court filings. "It necessarily follows that requiring the states to apportion approximately the same number of eligible voters to each district is the only way to enforce that constitutional right."

Should the Supreme Court rule in favor of the plaintiffs, many state and municipal governments nationwide would be forced to redraw district lines. Civil rights groups fear that people in urban districts with large numbers of non-voters would be disenfranchised. Tens of millions of children, prisoners, ex-felons and people with intellectual disabilities would no longer be counted because they cannot vote. This includes as many as 55 percent of Latinos, 45 percent of Asian-Americans and 30 percent of African-Americans, according to Time.

Historically, Democratic urban districts with large numbers of non-voters would grow larger, but there would be fewer of them, according to USA Today. Suburban and rural districts that are largely white and often Republican would be smaller and more numerous.

Ultimately, if the Supreme Court agrees that district lines should be drawn according to the number of eligible voters, Republicans would benefit and Democrats would lose.

Texas argues that the states should have the right to decide how to draw districts, and President Barack Obama's administration agrees. "States are permitted to use total population when they reapportion, and they are also entitled to choose citizen or voting-eligible population," argued Texas' lawyers in briefs. "This choice does not violate the Equal Protection Clause."

Penda Hair, co-director of the civil rights group Advancement Project, said in a statement: "To reverse course now is to shun precedent and needlessly engineer a plan to bring harm on a specific subset of people; namely communities of color — including children — undocumented individuals and families, persons with prior felony convictions and individuals with disabilities."

Almost all states and localities currently count everyone, and congressional lines would not be affected, as the Constitution requires that House seats be apportioned based on total population.