Trump's strategy is to immediately counterattack when attacked, and to not let his opponents get a second wind. This is no different.
Trump should point out, relentlessly, how Mitt Romney trashed him when he viewed it expedient to do so, and sucked up to him in other circumstances when it seemed expedient to do so. This says more about Mitt Romney than it does about Trump. Romney called Trump a "con man" and a "carnival barker" in early 2016, but after the 2016 election, he had lamb chops with Trump in a pricey restaurant and pretty much begged to be Secretary of State. Somehow, Mitt Romney isn't winning me over on his "character" arguments.
Why the hell do you need Romney to assist you with ascertaining trump's "character"?
It is blatantly obvious that trump is a vile, narcissistic, man-child of a human being.
PS: You are parroting trump with your "begged to be Sec of State" comment, so don't try to sound like you are telling us something new. (That was in trump's tweet.)