If Ds win the Senate in 2018, probability it was due to an "out of class" flip?

[Nichlemn]

It's January 3rd, 2019. All you know is that the Democrats control the Senate today. Given that information, what's the probability they have gained one or more seats that were not in Class 1 (the Senate class up for election in 2018) since today, and this was essential for their control?

In other words, how critical is it for the Democrats' chances of winning the Senate that they get a surprise seat?

[Nichlemn]

As in, Cory Gardner or Marco Rubio suddenly resigning out of nowhere? I guess that's a possibility. I'll go with 60% since, while I think it's POSSIBLE that Democrats will get one of UT/TX/NE/MS/WY/TN, it's unlikely that Democrats will do that while still retaining all current seats and picking up NV and AZ.

Not to be too morbid about it, but I think John McCain or Chuck Grassley dying suddenly is the more probable outcome.

Also, I could definitely see Collins flipping in a 50/50 Senate if she's fed up enough with Trump.