There are a few basic rules for the shape of congressional districts. They’re supposed to be contiguous and as compact as possible, for example. They’re supposed to try to preserve other political subdivisions, such as county lines. But the more difficult part is making sure that districts comply with the Voting Rights Act (VRA), which requires that district boundaries be drawn to ensure that black voters have electoral influence, even though the boundaries can’t be drawn primarily based on race.

Putting more minority voters into a district gives those voters more political power, but only in that district. So a map that groups too many minority voters into a few districts limits their electoral power by confining it to a small number of districts.

When the Supreme Court decided Georgia v. Ashcroft in 2003, it determined, essentially, that rather than having a few majority-black districts, the black community could be better served by having more state Senate districts where African-Americans constituted a high percentage of the population but less than a majority. In other words, ensuring that black voters have “electoral influence” doesn’t necessarily mean having a few overwhelmingly black districts. In theory, black Democrats could represent a majority of primary voters — but less than a majority of all voters — then team up with white Democrats in the general election to elect whomever was selected in the primary.

Now, however, it’s much more difficult for black voters in the South to find enough allied white voters to elect the representatives they want (almost always Democrats). There are just so few white Democrats left in the South, especially in the Deep South, compared to 10 years ago. It’s made drawing maps that meet VRA standards trickier. This problem is at the heart of two Congressional gerrymandering cases the Supreme Court heard on Monday, one from North Carolina and one from Virginia. The Court will have to decide how the VRA applies now that race has become such a strong proxy for partisanship in the South.

We can see the abatement of white Southern Democrats in the Cooperative Congressional Election Study (CCES) data set. The poll, which started in 2005, surveys over 50,000 people nationwide every two years, providing more accurate demographic and geographic subgroup data than other publicly available surveys. The CCES also verifies the voting status of all its respondents to see if they’re registered.

In the 2006 House elections, Republicans won Southern white voters by 16 percentage points, 58 percent to 41 percent, according to the CCES. That may seem like a wide margin, but Democrats did well enough with white Southerners to win in several majority-white districts. Democrats like Lincoln Davis and John Tanner won in Tennessee. The majority of Arkansas’s delegation was Democratic.

Of course, 2006 was also a very good year for Democrats; they won the national House vote by 8 percentage points. So as our baseline, let’s look at how Republican-leaning the Southern white vote was relative to the national House vote. In 2006, Republicans lost the national House vote by 8 percentage points but won white Southerners by 16 points. The relative difference gives us the lean — white voters in the South were 24 percentage points more Republican-leaning than the country that year.

The 2014 election (the last for which we have CCES data) was a much different story. Southern whites voted Republican 70 percent to 28 percent — a margin of over 40 percentage points, more than double the GOP’s margin among white Southerners in 2006. Once you take into account the fact that Republicans won the national House vote by 6 points and that the overall CCES sample was slightly more Democratic than the actual vote, Southern whites were still 39 percentage points more Republican-leaning than the nation. That’s a 15 percentage point jump from 2006. To put that in perspective, people are making a big deal over some Midwestern states — Michigan, for example — going from slightly more Democratic than the nation to slightly more Republican between the 2012 and 2016 presidential elections. What’s occurred in the House over the past decade is a political earthquake by comparison.

This difference makes it much more difficult to elect a Democrat in the South now — you need many more minority voters to do it. Given how nonwhite people voted compared to the nation as a whole in 2014 (45 percentage points more Democratic), the average district now needs to be less than 54 percent white in order to elect a Democrat in a neutral year. (According to the CCES, whites make up 69 percent of all voters in the South.) If whites were still as Republican-leaning as they were in 2006, the average district could be as much as 65 percent white and still expect to elect a Democrat in a neutral year.

When we focus solely on black voters — rather than nonwhite voters generally — the shift is just as dramatic. Southern black voters were 77 percentage points more Democratic than the nation in 2014. In a hypothetical district with only white and black voters, whites would need to make up less than 67 percent of voters to elect a Democrat in a neutral year. If whites voted the same way they did in 2006, a district could be about 77 percent white and would elect a Democrat, on average.

That’s a tremendous difference from a decade ago. That means the VRA is once again becoming increasingly important to Southern black voters’ ability to choose their representation in Congress. Although districts don’t need to be majority black, they need to be far closer to it.

This is all readily apparent if you look at who is actually elected to Congress. The number of non-Hispanic white Democrats elected to represent Southern states has dropped precipitously. To make a comparison across time, I’ve used the 109th Congress (elected in 2004) and the 115th Congress (elected this year). The national House vote — and therefore the number of seats controlled by each party — in those two cycles is pretty similar, making for a more apples-to-apples comparison than comparing the 2006 and 2014 results would be.

WHITE DEMOCRATIC REPRESENTATIVES ENTERING THE… STATE 109TH CONGRESS (2005-2007) 115TH CONGRESS (2017-2019) CHANGE Alabama 1 0 -1 Arkansas 3 0 -3 Florida 4 5 +1 Georgia 2 0 -2 Louisiana 1 0 -1 Mississippi 1 0 -1 North Carolina 4 1 -3 South Carolina 1 0 -1 Tennessee 4 2 -2 Texas 3 3 0 Virginia 2 2 0 Total 26 13 -13 White Democrats are disappearing in the South Based on data from The Cook Political Report, Candidate biographies and media reports

Overall, the number of white Democrats representing Southern states has been cut in half, even as the region picked up seven additional seats because of reapportionment after the 2010 Census. Florida is the only state where there are more white Democrats coming into Congress now than there were in 2004. In eight of the 11 Southern states, there are fewer.

The drop has been most drastic in the Deep South (Alabama, Georgia, Louisiana, Mississippi and South Carolina). Despite being home to less than 30 percent of all Southern white voters per the CCES, nearly half of the loss of white Democrats in the South since 2004 comes from these five states.

But let’s go back to the voters — the number of white Democrats in Congress from the Deep South is merely reflective of the voters themselves. If the broader South has seen a decline in the Democratic white vote , the Deep South has seen an absolute collapse.

In 2014, Democrats lost the white vote in the Deep South by about 65 percentage points, 81 percent to 17 percent. That made white voters in the Deep South 61 points more Republican the country as a whole. In 2006, whites in the Deep South were heavily Republican, but Democrats stood a fighting chance. Republicans won Deep Southern whites by about 30 percentage points in the 2006 House elections. Even taking into account how good of a year 2006 was for Democrats, whites in the Deep South were 38 percentage points more Republican than the nation as a whole. That was good enough for Democratic representatives like Bud Cramer to win in Alabama, Gene Taylor to win in Mississippi and John Spratt to prevail in South Carolina.

As white voters in the Deep South have shifted decisively towards the GOP, politics in the region have become entirely racially polarized. Democrats won nonwhite voters in the Deep South 80 percent to 18 percent in 2014 — the group was 65 percentage points more Democratic-leaning than the nation as a whole. So race has become an almost perfect proxy for partisanship in the region. As a result, a district needs a lot more nonwhite voters to have a chance of electing a Democrat in a neutral year. In the average Deep South district, a district must be less than 52 percent white for a Democrat to have a better than 50-50 chance of being elected. Per the CCES, white voters make up closer to 65 percent of voters in the Deep South as a whole.

Now there are no white Democratic representatives left in the Deep South. Cramer retired and Spratt and Taylor both lost re-election bids in 2010. John Barrow from Georgia was the last white Democrat in the Deep South to win a seat in the House, but even he went down to defeat in 2014. He lost in a district that was only 57 percent white.

Barrow would likely have held on if the dynamics of 2006 were still at work. If Deep South white voters were still just 38 percentage points more Republican-leaning than the nation — as they were in 2006 — the average district could be 63 percent white and elect a Democrat. In other words, the average Deep South district could have demographics roughly proportional to the region as a whole.

The changing landscape of race and partisanship in the South makes the Supreme Court cases that much more complicated. It used to be pretty easy to balance minority power in Congress and district compactness. The math has changed. It could, of course, change again. Georgia, for instance, has seen a shift toward the Democratic Party at the presidential level over the past few cycles (after swinging towards the GOP in the late 1990s and early 2000s). And while white Southerners’ shift toward Republicans was part of a long-term trend, much of the dramatic shift we looked at here occurred while the first black president was in office. With President Obama’s tenure ending, perhaps the trend will partly reverse itself. As long as race and partisanship are so closely linked in the South, however, it’s not clear how state legislatures and the courts can ensure that minority voters have political power without narrowly constraining where they exert that power.