I'm going to make a bold, bold prediction: Charlie Cook's initial rating for this race will be "Toss Up".
Also, he won't change it in any direction until at least mid-2016 barring some kind of freak wave emerging before then. Good chance it stays at "Toss Up" all cycle, even if Kirk wins or loses by 10.
Cook needs to remodel his ratings. Either pull a Sabato and call every race by election day, or split up the toss-up category like Rothenberg does (Pure Toss-Up, Toss-Up/Tilt D, Toss-Up/Tilt R).
I just went and checked his last ratings for 2014, man they were bad. Pryor and Landrieu "toss ups"? Seven toss-ups total? Michigan "Lean D"? I'm sure he'll convince himself he did well because he didn't "miscall" any races and the ordinal rankings of competitiveness are basically right. But pretty much anyone could do that.
It's really best to look at Cook's Current Ratings like this:
Solid: Barring unforeseen circumstances, this race will go for one side or the other.
Likely: Barring foreseeable but unlikely circumstances, this race will go for one side or the other.
Lean = Sabato's Likely
Toss-Up = Sabato's Lean/Toss-Up combined.