A map showing hypothetical new 2nd and 5th Congressional Districts in Florida (click to enlarge)
For those wondering why so many observers are confident that Florida's 2nd District will get redder following the upcoming round of court-ordered re-redistricting, the rationale lies in the court's decision regarding the 5th District, as Stephen Wolf explains. Right now, the serpentine 5th stretches across a 200-mile run from Jacksonville to south to Orlando; the state Supreme Court recently determined that this was an impermissible gerrymander designed to benefit Republicans by packing too many black voters (who lean heavily toward the Democrats) into a single seat—and thus keeping them out of surrounding seats. That contravened the Fair Districts amendments that voters added to the state constitution in 2010 and which form the basis of the court's ruling.
However, the 5th District is also over 40 percent black, meaning it's home to enough black voters such that they're able to elect a candidate of their choice—African-American Democrat Corrine Brown has long held the seat—which in turn means that the district is protected under the Fair Districts amendments.
To maintain a sufficient black population for the 5th District to keep sending a black Democrat to Congress (though likely someone other than Brown), the only more compact alternative to the current vertical configuration is a horizontal district that stretches from Jacksonville to Tallahassee, which has a sizable black population of its own. (Indeed, the court ruled that the 5th "must be redrawn in an East-West manner.") Wolf concludes such a seat would be similar demographically to the current setup; any other alternatives simply wouldn't contain an adequate number of black voters.
In addition, dismantling the southern portion of the 5th would likely turn some Republican districts bluer, as black voters, previously packed into that Orlando tendril, would now get spread across other seats. But it would also cause a major ripple effect in the 2nd District, currently held by Rep. Gwen Graham, because Tallahassee forms its Democratic core. Without Florida's capital city, just about any plausible version of the 2nd would become much more Republican.
One possibility, drawn by Matthew Isbel, is shown at the top of this post; he estimates that the district shown in red would have given Brack Obama just 35 percent of the vote, a steep drop from the 47 percent he won under the current lines, and enough to put it out of reach for Democrats under any set of circumstances. That's why Democrats are so down on the future of this district, and it's why Graham isn't ruling out a bid for Senate.