The Supreme Court's "one person, one vote" case, Evenwel v. Abbott, is theoretically easy because nobody can seriously argue that large disparities in eligible voters is unproblematic. If you have two districts with roughly equal populations but with 10 voters in the first and 100 in the second, that's a problem: Votes in the first district are worth 10 times those in the second.
It's the same issue that the Supreme Court faced in Reynolds v. Sims — the 1964 case that established "one person, one vote." It occurs in other countries as well: The British Parliament had to pass the Reform Act of 1832 to remedy the "rotten borough" situation, in which some members of the House of Commons represented thousands of people while others represented literally nobody.
When you cast a vote, you don't expect that your vote may count for less than someone else's merely because of where they live. But that's what Texas is trying to do: The state drew its districts to equalize total populations, ignoring how many of those people are eligible voters. (This is what nearly all states do, though 10 have constitutional or statutory provisions that exclude particular groups — such as foreigners, military personnel and inmates — from districting calculations.) The result is a plan that would create one Senate district with 388,000 eligible voters and another with 611,000. Those in the first district would have votes that count for double the votes of those in the second.
The 14th Amendment's Equal Protection Clause — "no state shall ... deny to any person within its jurisdiction the equal protection of the laws" — doesn't require absolutely equal districts, but surely ratios approaching 2-to-1 go beyond even the constitutional wiggle room allowed by the Reynoldsprecedent. If a state really only has to care about total population, it could theoretically create districts in which only 10 percent, 5 percent or even 1 percent of residents were eligible voters — and those tiny groups of voters would each be able to choose one representative all the same.
Texas is arguing that it should be able to draw its voting districts based on total population because that's how the Constitution apportions seats in the House of Representatives. But that so-called "federal analogy" is the same losing argument that Alabama used in 1964. Alabama defended in Reynolds a system that gave one senator to each county, which mirrored the U.S. Senate's treatment of states and resulted in similar population disparities.
The Court rightly rejected that logic, arguing that "the federal analogy [is] inapposite and irrelevant to state legislative districting schemes." In other words, just because the federal government does it, doesn't mean the states can. After all, the states are "separate and distinct governmental entities which have delegated some, but not all, of their formerly held powers to the single national government," while state districts aren't sovereign entities.
Today, the Court is presented with a new version of the federal analogy. Texas argues that because the Constitution allocates House seats by total population, states should draw districts that do the same for their own legislatures.
This version of the federal analogy works no better than the last one. Section 2 of Article I of the Constitution and Section 2 of the 14th Amendment show that the total population standard should apply to separate states that possess legal autonomy otherwise — not least in defining for themselves how to select their members of Congress.
Moreover, since states define voting rights for themselves, a rule based on eligible voters would provide states with a perverse incentive to expand suffrage as much as possible (for example, by lowering voting age to 12) and thus artificially acquire more representatives. But a county can't lower its voting age in a bid to gain more state senators. The primary justification for the federal rule — preventing states from "gaming" congressional apportionment — simply doesn't exist at the state level.
Indeed, the true federal analogy is to the part of the 14th Amendment that removes disenfranchised former slaves from their states' apportionment total, so as not to give more voting power in Congress to states that denied freedmen the right to vote. This provision confirms the principle that non-voting persons aren't "virtually" represented by their neighbors' votes, so they shouldn't be used to give those neighbors more power.
Federal apportionment therefore has no role in shaping how states create their districts. Under "one person, one vote," every eligible person should be able to vote and each vote should be equal.