Sen. Thune (SD)

[BM]

He's probably safe if Stephanie runs too, which is why she won't.  She's definitely looking at the governor race instead, but is probably even a little wary of that.

[BM]

That's her best option, followed by running in the open election for governor. Running for Senate against a relatively popular incumbent in what's likely to be a mildly pro-Republican environment would really make no sense.