Our team ran 10,000 simulations using our most recent ratings of the 36 seats up for grabs on Nov. 4. It showed Republicans with a 50.03 percent chance of winning the Senate and Democrats with a 49.97 percent chance of holding the Senate. Again: pure toss-up.
What all three models agree on? The GOP is strongly favored to win four seats -- South Dakota, West Virginia, Montana and Arkansas -- and also has pretty good odds in a fifth: Louisiana.
From there, so long as Republicans hold on to Kansas (which is a weird case all it's own), all three models currently see Alaska and Iowa as pivotal for the majority of the Senate. The GOP wins one of them, and it has the six seats it needs.
Here's how each model rates those races:
And now you can see why the battle for the Senate is 50-50. Republicans are favored in two of these six scenarios. If Democrats were to sweep both seats, they'd probably be returning in January 2015 with their majority intact -- albeit barely.