On Tuesday, the U.S. Supreme Court will hear one of the most politically consequential cases in years, to decide whether partisan gerrymandering, or having elected politicians choose which voters do and don’t cast ballots in specific U.S. House and state legislative elections, is constitutional.

If you want to know why the GOP has not only controlled the House but has supermajorities in states that should be politically purple, such as North Carolina and Georgia, the answer is extreme partisan gerrymandering.

If you want to know why House Speaker Paul Ryan cannot control his most right-wing members, as exemplified by the House Freedom Caucus (which didn’t think Ryan’s bill gutting the Affordable Care Act went far enough), the answer is extreme partisan gerrymandering.

If you want to know why the Democrats face such a steep climb in 2018 to retake the House (because they need 24 seats and there aren’t dozens of competitive races), the answer is extreme partisan gerrymandering.

If you want to know why so many red states are passing voter suppression and anti-abortion laws, blocking LGBTQ rights, and sued to block Obamacare and climate change-related environmental protection laws, the answer is extreme partisan gerrymandering.

“Through redistricting, Republicans have built themselves a perhaps unbreakable majority in the House,” Ryan Lizza, the New Yorker’s Washington correspondent, wrote in 2013 about the House Freedom Caucus. “But it has come at a cost of both party discipline and national popularity. Nowadays, a Sunday-school teacher can defeat the will of the Speaker of the House.”

The Supreme Court’s case raising whether extreme partisan redistricting is constitutional comes from Wisconsin. There, despite only 23,000 votes separating Donald Trump and Hillary Clinton in November 2016’s vote, Republicans hold two-thirds of the seats in its state legislature and House delegation. How did they get that power? It's a pattern also seen in Michigan, Ohio, Florida, Texas, Georgia, North Carolina and Virginia—all states tipping the balance in national elections in recent years.

The answer is by segregating reliable Republican and Democratic voters, so Republicans could win on Election Day by smaller percentages than what Democrats would win by in their strongholds. That comes from drawing election districts so there were more reliable Republican voters in more districts, and crowding Democrats into fewer districts in their states.

Political district maps are redrawn after the once-a-decade federal Census. The result of the GOP’s extreme partisan mapmaking in 2011 has been its lock on the House, as well as on the red states that subsequently fought all things Obama and passed anti-abortion laws. These states have gone on to adopt the GOP voter suppression catalog to block Democratic voters, including tougher voter ID requirements to get a ballot, ending Election Day registration and voting, purging infrequent but otherwise legal voters, ending early voting, pre-registration of teens, etc.

In 2010 and 2011, Obama and the Democrats weren’t focused on blocking extreme redistricting and underestimated the GOP response to Obama and the Democrats’ 2008 landslide, which became one of their biggest errors. While Democrats today are waving the “never again” flag, their best hope for returning the political system to one based on competitive elections lies with the Supreme Court—which is hardly worth banking on. The Court has never invalidated political maps for arch partisanship; they only invalidate maps that are racially based to disenfranchise non-white voters.

The best way to understand how extreme redistricting works, and why it is so impactful comes from a Supreme Court decision this past spring, where the Court—before Judge Neil Gorsuch was seated—ruled that North Carolina’s congressional maps were illegal racial gerrymanders. In that decision, the Court’s majority noted that most Republican House members had been elected with 56 percent of the vote, while the state’s few elected Democrats won their seats with nearly 70 percent of the vote in their districts.

That figure—56 percent—is key. It’s in synch with what many election data analysts say is the built-in starting line advantage that the GOP achieved via redistricting: a 6-point head start. Needless to say, that is not all the GOP in these red states has done to disenfranchise Democrats. Stricter voter ID pre-empts another 2-to-3 percent of likely Democrats, according to academics, just as making voter registration tougher and narrowing voting options, like early voting, shaves off more fractions of a percent to tilt likely outcomes.

The Wisconsin case that prompted the Supreme Court review came after a lower court ruled that the maps drawn by its GOP in 2011 created so many wasted votes by Democrats that Wisconsin elections were anti-democratic. The Wisconsin GOP appealed and the high Court took the case.

What’s likely to unfold on Tuesday in the Supreme Court are exceptionally technical arguments about how to measure extreme partisanship and unfair, uncompetitive elections. Several years ago, Justice Anthony Kennedy, in another redistricting case, wrote that he wished that there was an objective standard. That prompted the legal team challenging Wisconsin’s maps to create a metric based on wasted votes.

But in the Court’s ruling last spring on North Carolina’s unconstitutional racial gerrymander—without Gorsuch—the Republican-appointed justices, including Kennedy, said in a dissent that partisanship was a part of human nature and politics. It might be odious, they said, but they were averse to regulating it. Whether that view holds is the critical question behind the Tuesday hearing at the Supreme Court.

But no matter what emerges, all Americans should note what extreme gerrymandering has done to the nation’s political culture and process.

It has ended competitive elections, boosted the most extreme Republicans in the South and Midwest, and left the nation split in two—with Democratic-run states on the coasts and vast red middle America. In numerous states, like Florida, there are cities with progressive mayors while legislatures and congressional delegations are deep red, and fervent political opponents.

Ultimately, extreme gerrymandering has undermined representative government, shrunk the role of citizens and empowered extremists—whether President Trump or the House Freedom Caucus. It’s allowed Republicans to create a structural starting-line advantage of nearly 10 points, regardless of candidates and issues. That has produced a nation where Democrats might win the popular vote but don’t win political power.