The irony of course is that the ACA is closely modeled after Republican plans, both Romney's plan in Massachusetts and a Heritage Foundation proposal from way back in 1989 (here http://thf_media.s3.amazonaws.com/1989/pdf/hl218.pdf).  If the ACA had never occurred and Republicans were asked to come up with a new plan, it would probably look a lot like a scaled-down version of the ACA (the ACA has improved on these earlier models, for example it says there can be no lifetime caps in coverage, it requires that insurance companies spend no more than 20% of premiums on admin costs, parent's plans cover kids until they turn 26, it provides free preventive care, and some other benefits).

But now the Republicans have pledged to repeal the ACA and Trump has promised that their replacement plan will cover everybody (that's millions more than the ACA) and cost less. I can understand why they are fretting; none of their current proposals come close to these goals.