He's more likely to screw the Dems than the GOP if Gary Johnson 2016 is any guide. An anti-establishment Indie trying to out populist Trump would hurt him the most.
Gary Johnson took more GOP voters than Democrat. Gary Johnson voters voted downballot Republican by a margin of 20% more than they did Democrat.
Jill Stein, however, probably took more Democrat voters than GOP.
Yes, but they were the quintessential Clinton-Republican demographic. Do you really think Johnson voters in PA-06, NC-13, AZ-09, etc. would have broken for Trump? Johnson gave disgusted Romney voters an out and prevented Clinton from swinging enough of them in enough key places. He's probably the single greatest reason Trump won.