Graham said it plainly, the fear is that the precedent will allow ICC to go after US next. While US doesn't recognize the ICC, this will be a geopolitical nightmare for the US. For example, countries that do recognize ICC, which is most of Europe, would be required to arrest US politicians when they visit.

https://thehill.com/video/sen-lindsey-graham-if-icc-does-this-to-israel-they-will-come-for-us-next%e2%80%99/9718460/