10 points behind Trump, because Chafee is a nice but fairly stupid man and does not contrast well with Trump's bullying.
I'd think voters would be even less impressed than they are with their current choices; you'd be dealt the unpleasant experience of watching, well, Trump, go up against a guy who in any vaguely ability-based system would be having the living stuffing beat out of him for his lunch money. It'd be the biggest matchup of "lightweights" since... well, ever, really. The only comparable race I can think of would be the 1896 Vice Presidential election.
Chafee was the lamest of the Democratic candidates, but if he were the candidate, Trump would have been running a different campaign. He'd be calling Chafee a "baby" and a "weakling", but he wouldn't really be able to hit him with anything concrete. There are concrete things to hit Hillary with in this campaign that stick.
