On Form I864, petitioners must prove that they have the ability to financially support the visa applicant(s) if necessary. How Can I Prove U.S. Domicile?This guide will clarify what domicile is, how to establish it, and the documentation you need for proof of domicile. A declaration is a simple statement that you intend in good faith to return to the United States to resume your domicile. If you want to sponsor someone for a family-based green card, you need to provide proof of a U.S. domicile in your Affidavit of Support (Form I-864). One of the requirements for Form I864 is showing that you are domiciled in the US. In other words, your principal residence must be located in the US.