It’s hardly equivalent if we’re talking about running mates. Warren isn’t anybody’s running mate. If she somehow ends up as Hillary’s VP pick (she won’t - they do not like each other AT ALL), then of course he should go after her.
Well then make it a Bernie/Warren ticket, and Trump going after Hillary.
Heck, Trump goes after Bernie directly, that costs him some Bernie voters already. And if Bernie is on the ticket, is Trump supposed to lay off him?
As it stands, we can only guess who the DEMs will run.
Warren went after Trump. Is he supposed to just take it?