Florida's state Supreme Court struck down much of the state's congressional map Thursday, ruling 5-2 that eight districts were gerrymandered to favor Republican candidates. In most of the US, partisan gerrymandering is perfectly legal. But Florida voters recently approved a constitutional amendment that banned drawing districts to favor parties or incumbents. Even after the amendment was approved, though, it was unclear whether state courts would enforce its language very strictly, or whether they'd defer to the legislature. Now this strong ruling shows that the courts will strike down maps they view as drawn with partisan intent. "The voters sought fair districts," the justices write. The justices ordered the legislature to draw a new map that would then have to gain court approval, and laid out various guidelines for how the process could be done more transparently.

Florida voters banned partisan gerrymandering in 2010 — yet Republicans kept winning tons of the state's congressional seats

In 2010, Florida voters overwhelmingly approved a constitutional amendment banning partisan gerrymandering. The strongly worded amendment, which passed with more than 62 percent support, specifically said that districts "may not be drawn to favor or disfavor an incumbent or political party."

Yet Florida's map-drawing process was still left to the state legislature, then controlled by Republicans. And when they redistricted shortly afterward, the maps they came up with seemed to many to be slanted in favor of the GOP. Indeed, in 2012 Republicans got 51 percent of the US House votes — but they ended up with 63 percent of the seats, 17 out of 27.

There are potentially other factors that could explain a pro-GOP slant in House districts. But the League of Women Voters, several media organizations, and some voters saw partisan motivations at play — arguing that in many instances, the public redistricting process was a sham, with the real decisions being made by partisan Republican operatives behind the scenes. So they filed suit to try to get the map struck down.

Now a five-vote majority of the Florida Supreme Court has ruled against the legislature. The majority includes two justices appointed by Gov. Charlie Crist (who was then a Republican), two appointed by Gov. Lawton Chiles (D), and one whose appointment was the result of an agreement from Chiles and Jeb Bush. Two justices dissented, both of whom were appointed by Crist.

The court has now ordered eight districts to be redrawn

The state Supreme Court did not throw out the whole map, but rather ordered that eight specific districts — those circled in red above — be redrawn, as well as all other districts affected by those new borders.

Check out in particular the fifth district, held by Democrat Corrine Brown, in the northern part of the state:

Frequently referred to as "serpentine," it contorts and stretches vertically to scoop up several urban centers across Florida. This is a typical sign of gerrymandering, indicating that many voters who tend to support Democrats are being artificially packed into one unusual district, to dilute their voting power.

Republicans in the legislature claimed that that district was only drawn that way to make it a majority-minority district, to comply with the federal Voting Rights Act, as the state Senate president testified. However, the Senate GOP's lead redistricting expert cast doubt on this claim when he testified that putting more African Americans in the district was "not a necessity."

Now the state Supreme Court has concluded that the legislature "failed to meet its burden to demonstrate" that the fifth district "passes constitutional muster," and has ordered that it be redrawn in "an East-West orientation." It ordered seven other districts redrawn too, for a variety of reasons.

The state Supreme Court ruled that evidence of partisan "intent" can be used to challenge a map

As the trial was going forward, Republican legislature representatives argued that they had carried out a transparent and nonpartisan process, featuring many public hearings, credentialed experts, and publicly released drafts of maps.

But the plaintiffs argued that GOP operatives were influencing deliberations behind the scenes in a "shadow" process, for partisan gain. For instance, one Republican staffer gave draft maps to a party operative weeks before they were made public — and the party operative worked on new some maps that were strikingly similar to the final legislature-approved plan. Another party staffer appears to have drawn certain district maps that were later officially submitted to the legislature under someone else's name.

As the first-of-its-kind trial went forward, it was unclear whether claims like these — which relate to the legislature's motives, rather than its results — would carry much water in the courts. Indeed, the trial court that first found the map unconstitutional decided to focus mainly on objective indicators of whether its districts were rigged for partisan purposes. It relied on assessments from prominent political scientists, like Jowei Chen from the University of Michigan and Jonathan Rodden of Stanford, who created a computer simulation finding that it was "virtually impossible" for Florida to produce such a pro-GOP map through a nonpartisan process. (Nolan McCarty of Princeton disagreed, writing in a court filing that their simulation "does not and cannot account for all of the factors that real-world mapmakers must consider.")

But now the state Supreme Court has gone much further, ruling that evidence of partisan "intent" in redistricting can indeed be used to challenge the legislature's plan. The justices ruled that "once a direct violation of the Florida Constitution’s prohibition on partisan intent in redistricting was found, the burden should have shifted to the Legislature to justify its decisions in drawing the congressional district lines." They continued: "A finding of partisan intent therefore renders the Legislature’s redistricting plan constitutionally invalid."

The court has set forth guidelines for a more transparent process

The existence of a "shadow" redistricting process last time around spurred the court to set down guidelines for how these districts should be redrawn. They included:

That all meetings where decisions about the maps are made should be public

That all non-public meetings about the maps should be recorded for preservation

That there should be a mechanism for people to submit alternative maps, and for citizens to offer feedback

That all emails and documents related to drawing the map should be preserved. (Many were deleted last time around.)

That the legislature publicly document its justifications for its new district lines

The justices write that they "understand that 'taking the politics out of politics' is itself a difficult challenge," but that "the voters sought fair districts." They argue that they now have a "solemn obligation to ensure compliance with the Florida Constitution" in this regard.