If Republicans win the trifecta (FL/OH/PA), NC might become a must-win for Democrats.
I'd inverse that statement.
Yeah, if Republicans win all three of the "trifecta" then they've probably already won NC and AZ. If Republicans win NC, AZ (and MO and IA) then Democrats need all three of FL, PA, and OH. After Grassley and McCain, I'd say Burr is the next strongest incumbent of the states I listed.
Honestly, I think Toomey is in better shape than Burr. Burr has been in for 12 years and has been pretty invisible - has always had mediocre approval ratings and while I don't expect the race to change much, a 3 point lead over a mediocre opponent isn't good. Toomey has been a solid senator and has good favorability ratings - he hasn't trailed in a single poll so far and the last one had him by 9.
That being said, I've been following Phil's FB for the past few years, so I may be a bit biased/optimistic for Toomey.