What Romney really means is ‘I’d like to hear from McGahn under oath that Trump asked him to lie’ (which would start impeachment proceedings)
Yes, he would. Which is why Mr.McGahn as instructed by his former boss, the President (more precisely the DOJ presidential apparat) will order him NOT to speak to the Congress, under the proviso that ANYTHING he would be asked to speak on— would be under Executive Privilege (and under orders thus) not to be discussed, leaked or anything else.
So, unless- as in the case of John Dean (nixon’s Mcgahn/counsel) unless the dhims have nasty stuff on McGahn (which is always possible under the deep state “collections” made for just this occasion- which can also be “declassified and exposed”) as blackmail, he will not testify, even under threat.