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). If you're planning to petition a family member for a green card, you'll most likely need to prepare Form I864, Affidavit of Support.This guide will clarify what domicile is, how to establish it, and the documentation you need for proof of domicile. These additional financial sponsors also have to submit an Affidavit of Support, proof of their income, and proof of their legal status in the United States. This guide will explain how to prove domicile when filling out Form I-864 ("Affidavit of Support"). Consular processing support without the high price tag. Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States. Contact the Treasurer's Office or Chief Fiscal Officer of your home county to inquire about acceptable forms of proof you will need to prove your residency.