LOL Byrd, Feinstein and Kennedy before Nelson? Whoever created this list is on crack.
Unfortunately, they will be lucky to find some underfunded nobody or some rich guy who can pay for his own campaign to run against Byrd and Kennedy because it's hopeless. These two will be carried out of the senate feet first. The machinery is in place to re-elect them even if both are in a coma and can't campaign.
I don't believe Bill Nelson of Florida is vulnerable because he runs to the center and has voted with Republicans a lot. I would depend on who the Republicans nominated. If it was Jeb Bush, then yes, Nelson would be very vulnerable.
The only way Byrd or Kennedy are going to be defeated is through death. Considering their age, that is very likely.
Perhaps the poster thought that Robert C. Byrd might retire to spend time with his dog "Billy" before Billy dies!!!!
Likely to be defeated?...Kennedy,Feinstein,Byrd<? What have you been smoking. The Peoples Republick of Massassachusetts and California have spoken.