How is his lead bigger with likely voters than registered voters? If anything, I would expect the opposite.
Democrats are more excited to vote. 99% you can blame that on Trump.
That seems to be the case here, this time. Feingold supporters probably didn't turn out for him in 2010, but are excited to get him back it seems. Outside groups need to write this race off. This seat (and Illinois and New Hampshire as well) is gone.
I wouldn't write off Illinois just yet. And New Hampshire is most certainly NOT gone. Ayotte is tied with Hassan right now, while Duckworth and Feingold seem to have pretty decent leads. I'd expect Wisconsin and Illinois to go D this year long before New Hampshire.