Um no. Weld was running against an incumbent; this is an open seat in a less machine-oriented state.
Sanders is leftist but he quite simply is not a Democrat. Whether that winds up mattering is unclear, but certainly whatever organizational benefits come with being part of the DNC will not be there for him.
There actually is question who is more popular heads-up. Clavelle wasn't THAT bad and he was running a serious campaign; Sanders did not face real opposition. Also note that a Democrat in his House race fetched 7%, so I would expect anywhere from 5-8% to be ciphered in the Senate race.
If I had to bet even odds, I would take Douglas. If anyone is overrated here it's Sanders.
Sanders is basically the incumbent as he already holds a statewide Congressional position. And yes Douglas does too, but it's pretty obvious by looking at many races in the past to see that legislative and gubernatorial races are completely different.
If Sanders was so overrated and vulnerable, you would've expected the Republicans to try to beat him in the House, or at least put up a serious campaign against him.