On Monday, I spent about ten minutes filling out webforms pasting the same message to each of my representatives at different levels of government (one was surprisingly asshole levels of interaction, but I suspect that was incompetence rather than deliberate). I encourage you to do the same. If they can come for him, they can come for us. Feel free to borrow and modify:

Please publicly fight for the rights of Mahmoud Khalil. If they can come for him for having an opinion counter to AIPAC’s desires, they can come for US citizens. Thank you.