Expressions conveying (in an aphoristic way) the idea that gerrymandering means "politicians pick voters, not the other way around" are fairly recent. Book and newspaper database searches yield quite a few examples from the past 40 years. Although they do not agree on a set wording, they are very similar in tenor and effect, and they do seem to have the earmarks of an emerging proverbial phrase—short, catchy, and memorable. Here are some examples, from the period 1989–2005, in ascending order by date. The listing here is by no means exhaustive.
From an unidentified article in Outlook, volume 1, issue 3 (1989) [snippet view]:
It is actually the election process in reverse. In an election, the voters choose their politicians; but in redistricting, the politicians choose their voters. While citizens vote to influence the way in which they are governed, those elected redistrict to perpetuate their control and term of governing.
From Jamin Raskin, "Gerrymander Hypocrisy: Supreme Court's Double Standard," in The Nation, volume 260 (1995) [combined snippets]:
By fencing out unfriendly voters and potential rivals, incumbents make districts in their own image and turn elections into a formality. In our self-perpetuating incumbentocracy, voters don't really pick public officials on Election Day because public officials pick voters on redistricting day.
From Lani Guinier, Lift Every Voice: Turning a Civil Rights Setback Into a New Vision of Social Justice (1998) [combined snippets]:
Most of all, in a winner-take-all system of geographic districts, voters don't choose their representative. Representatives choose their voters. This means that candidates are reelected not because they mobilized supporters to go to the polls but because they used the power of incumbency to draw the election district lines around those supporters.
From David Lawrence, America: The Politics of Diversity (1999) [combined snippets]:
A helpful way to think about reapportionment is this: With elections, voters get to choose their policymakers. With reapportionment, policymakers get to choose their voters.
From Elaine Marshall (North Carolina Secretary of State), North Carolina Manual, 1999–2000 (1999) [snippet view]:
The Republican Party of North Carolina believes that our citizens should elect government officials rather than let politicians pick their voters through crafty technicians with a redistricting computer. We support reasonable, compact, congressional districts and legitimate single-member legislative districts which do not split counties as is mandated in Article II, Section 3, Subsection 3 of the NC Constitution.
(Ironically, the U.S. Fourth Circuit Court of Appeals has, in the past five years, ruled on multiple occasions that the maps of congressional districts that the Republican-controlled North Carolina legislature drew up in 2010 and 2017 were unconstitutional gerrymanders designed to maximize Republicans' election chances.)
From an unidentified item in Texas Forum on Civil Liberties & Civil Rights, volume 6 (2001) [quotation not visible in snippet window]:
As the saying goes, "In gerrymandered election districts, the voters don't choose their politicians—the politicians choose their voters."
From Claire Maude, "Bush's Values Appealed to Southern Voters, Expert Says," in the Stanford [California] Daily (November 5, 2004, pages 3–4):
Expressing frustration with a practice that takes power out of the people's hands, [professor Jamin] Raskin denounced the practice of partisan officials choosing district boundaries known as "gerrymandering."
"Voters no longer pick representatives, representatives pick voters," he said.
From Rachel Abbey, "Many State Issues Fail," in the [Kent, Ohio] Daily Kent Stater (November 9, 2005, pages 2 & 7):
Districts are redrawn every 10 years by government officials.
"The current system allows politicians to pick their voters, rather than letting voters pick their politicians," [Keary] McCarthy [press secretary for Reform Ohio Now] said.
Conclusion
Expressions to the effect that gerrymandering enables politicians to choose their voters, rather than allowing voters to choose their politicians, are very common in the print record over the past 30 years. It is hard to say who first formulated the idea, but Jamin Raskin was an early and persistent propagator of it.
Whether the saying qualifies today as a fully established proverbial expression is difficult to say, because it is still relatively new. But if gerrymandering persists (as seems likely), I would expect the aphorism to become a permanent part of the political landscape, as "If voting changed anything, it wouldn't be legal" already has.
