It would be very surprising to me if SCOTUS upholds Trump's birthright citizenship order. So far they have not shown a willingness to just blatantly reinterpret the constitution for political reasons, just a willingness to discard well-established precedent.
If they do uphold it, they'd be torpedoing their already flagging legitimacy. To me it would be proof that we have genuinely entered a new phase of history. My estimate of the likelihood of civil war would rise dramatically.