I've got this irrational(?) fear that if Brexit passes Trump will win in the fall.

Because if Brexit passes one of two things will happen:

1) The economy will go down. In the worst case scenario the initial seed of financial disruption from Brexit passing will trigger the China debt default chain and cause a total meltdown.

But whether it goes down a lot or little in response to which Trump will blame the Democrats for the economy sucking, which isn't fair but historically the party in office has always caught the flak for a bad economy regardless of the causes.

2) The economy will stay stable or improve.

In which case Trump will say "Hey look, Britain followed the lead of a bunch of fascist, racist, idiots [okay, probably not in those exact words] and it worked out for them, so it will work out for us too!"

Come on UK, you knew how to deal with these types under much more trying circumstances when they were living on the other side of the Channel. Don't give up just because they've become a homegrown threat! You were strong then, you can be strong now!