A number of states currently under full GOP control—including Wisconsin, South Dakota, Kansas, Michigan and Ohio—allow the governor veto power over district maps drawn by the state legislature. With less than four weeks to go before Election Day, Democrats are either in the lead or in close races to retake the governor’s mansion in those states.

Redistricting experts say that winning just this one office represents Democrats’ best opportunity to have a voice in the once-a-decade redistricting process that next rolls around in 2021—and block the GOP from locking in majorities for another 10 years.

“The Republican gerrymander at the state legislative level is even more solid than the gerrymander of the U.S. House,” David Daley of FairVote, an electoral reform group, told TPM. “So if Democrats want to have a seat at the table in 2020 redistricting, the best way in many otherwise competitive states that are friendly to Democrats requires winning the governor’s mansion.”

“Governors are key in this process, and can be a powerful check and balance on efforts to gerrymander,” the Brennan Center’s Michael Li concurred.

Good government and redistricting reform groups like former Attorney General Eric Holder’s outfit, the National Democratic Redistricting Committee (NDRC), are well aware of these stakes. In laying out their targets for 2018, the NDRC said its goal was to break trifecta control—GOP rule of both houses of state legislatures and the governorship—to make sure “Democrats have a seat at the table.”

“What we know about redistricting is that the most extreme maps always emerge when one party has total control of the process,” Daley said.

To illustrate just how important governors can be to these battles, Li brought up the examples of Wisconsin and Minnesota. The 2010 Republican wave election gave the GOP total control in the Badger State, while in Minnesota, Republicans won both houses of the legislature, losing the governorship, by a hair, to Democrat Mark Dayton. When the redistricting process came around in 2011, following the 2010 Census, the two states ended up with starkly different outcomes.

In Wisconsin, Li said, the GOP “went to town and drew one of the most aggressive gerrymanders that the country’s ever seen in the state assembly.” Republican Gov. Scott Walker signed off on the maps, which were subsequently challenged in a case that went all the way to the Supreme Court.

In Minnesota, Dayton “was able to veto a very bad Republican gerrymander, which kicked the process to the courts. And the courts drew a map which has been much more responsive to electoral shifts.”

Control of Minnesota’s House has flipped from Democrats in 2012 to Republicans in 2014 and could flip back to Democrats in 2018.

Wisconsin, meanwhile, has been safely Republican for the past decade. In 2012, Democratic candidates for the state assembly earned over 174,000 more votes than their Republican counterparts, but Republicans took 60 of the 99 seats.

“2016 proved so uncompetitive that half—fully 49 percent of those seats—went uncontested by either major party,” Daley noted.

The Midwest has been an epicenter of Republican control post-2010, but Democrats are currently leading in the governor’s races in Illinois, Michigan and Minnesota, as Governing magazine recently reported. Walker is also in trouble in Wisconsin, while Democrats are making a strong showing in other states like Kansas, Iowa, Ohio, and South Dakota where the governor has either veto control over or a key role in the redistricting process.

Michigan’s case is distinct. Voters Not Politicians, a citizens group, succeeded in getting a constitutional amendment on the November ballot that, if it passes, will take control of the map-drawing process away from state lawmakers.

The huge enthusiasm for that effort, which gained over 400,000 signatures, is a testament to how enduring the effects of Michigan’s 2010 gerrymander have been. Democrats won more total votes in state House races in 2012, 2014 and 2016, but failed to take back control.

As redistricting reform advocates are quick to point out, gerrymandering is not exclusively a Republican problem. In Maryland, where redistricting is actually run out of the governor’s office, then-Gov. Martin O’Malley worked with the Democratic legislature to secure a 7-1 Democratic congressional gerrymander in 2010.

At a Baltimore event in September, Democratic gubernatorial candidate Ben Jealous said he would push the balance to 8-0 if he’s elected.

Dan Vicuña, national redistricting director at Common Cause, said these sorts of winner-take-all-approaches represent a “very short-sighted” view of how the process should work. Where they have full control, Democrats and Republicans are prone to making the kind of power grab they couldn’t stomach if control changed hands.

“It’s in the best interest of both parties to really hedge their bets and come up with some sort of real reform effort,” Vicuña told TPM. “Be that independent commissions or something with a partisan balance that prevents one party from steamrolling the other.”

Assuming that those efforts don’t come to pass and the process remains fiercely political, a wave of Democratic governorships in red states could result in “deadlock” over maps, according to Vicuña. As in Minnesota, that could mean the process gets kicked to the courts, which Vicuña said have historically “hewed somewhat closely to the lines that the state legislatures have drawn.”

But after a decade of gerrymandering games and drawn-out legal battles, the courts have started turning to special masters who’ve ignored the legislatures’ suggestions entirely.

“I think the courts will be far less likely to be deferential to the districts that legislators are proposing and more likely to start from scratch,” Vicuña said.