When Trump was close to winning the Republican Primary, Romney released a statement condemning him so Trump trashed him.
After the election Mittens went to Trump to 'kiss the ring' thinking he could become Secretary of State. Trump had no intention of giving him that job so he became the go-to Trump critic on the GOP side.
Next, he needed to win a seat in Utah. So he tried to get Trump's endorsement and support, which he got (sort of) and once he was elected he turned on Trump again.
The Trump campaign has since branded Romney as a 'snake' who infiltrated the Republican party on behalf of the Democrats to throw the election against Obama and destroy the party (and Trump).
It would be a good idea to put someone like James Mattis on stage to give a speech, who is respected by both parties and most importantly the armed forces.
I don't see how the likes of Kasich and Romney are anything but a liability for the Biden camp.