He will gain voters if an avowed socialist is his oppnent
Not likely. I'm not talking about hardcore republicans. I'm talking about the middle. Sanders has a great way of talking to the middle. They don't care about the socialist tag because Sanders can eloquently take care of that. What they will care about is that Sanders will talk about getting jobs in a way that Trump hasn't. Let's look at the narrow victory states, less than 5%
Arizona - Likely to stay republican, but the right running mate from Sanders could swing it, but i doubt the Ds will try that hard.
Colorado - Moving into a solid blue state. Don't see any movement there
Florida - Who knows. I honestly don't know which way it will go. I'm gonna assume Florida could stay in the Rep side, but it depends on how hard the war on welfare goes.
Georgia - If Sanders pulls Abrams as his running mate, Georgia could swing.
Michigan - I think Sanders absolutely runs better for the Working Class American than Hillary does, can easily see this flipping back to blue.
Minnesota - Hillary won this, I don't see Bernie losing this but it is possible.
Nevada - Hard to say, I'm gonna assume it stays blue as it's the direction the state goes but who knows.
New Hampshire - Bernie beat Hillary in the New Hampshire primary, he should perform as well as her if not better but it could go the other way.
North Carolina - Likely to stay with Trump but who knows. Bernie could pull an upset.
Pennsylvania - Will be close, but with a candidate speaking to the working class, likely to go back to being a blue state but I expect it to stay a battleground state for sure.
Virginia - Moving to a solid blue state. I don't see it switching to Trump but who knows.
Wisconsin - Similar to MIchigan I think ti goes back to being blue with someoen speaking to the rustbelt about jobs.
What the Trump camp is hoping for was that the big swing from 2012 to 2016 was a movement to the right. Maybe it was. More likely than not though it was a slide step based on Hillary's legendarily bad campaign. If some of the states that Hillary lost go back to Obama era voting, Sanders wins comfortably. That's not including the close states in 2012 like Iowa, Ohio and Florida, but if he takes back Pennsylvania, Wisconsin and Michigan, keep in mind Bernie smoked hillary in Minnesota, beat her in michigan, and smoked her in Wisconsin. Only state that's at risk that he didn't beat her in was Iowa (which was likely a setup DNC victory) Pennsylvania (Bernie I don't think loses many dems in Pennsylvania) and North Carolina which was a big loss.
The thing that has to keep the Trump campaign uneasy is the chance that Georgia could become a blue state. 2018 saw a Republican Incumbent barely beat a Democratic challenger. That has to have members of the republican party scared.
Worst case scenario for the reps right now is that this party field whittles down fast. They want to keep it wide open and have the candidates keep attacking each other, gives them much more canon fodder. If this wraps up quickly with a Sanders victory, then don't expect anything less than a really hard fight for Trump.