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). How Can I Prove U.S. Domicile?Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States. Current lease, mortgage statement or notarized domicile form. This guide will clarify what domicile is, how to establish it, and the documentation you need for proof of domicile. Gather Necessary Documents: Prepare identification documents, proof of residency, and any required training certificates.