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). On Form I864, petitioners must prove that they have the ability to financially support the visa applicant(s) if necessary.Learn how to prove your U.S. domicile for a family visa with our stepbystep guide. Essential documents and tips to ensure visa approval. How Can I Prove U.S. Domicile? In this post, we'll explore examples of proof of US domicile and the documents required to establish proof of domicile. Typically, in order to claim US domicile, the Affidavit of Support sponsor must be residing in the United States. The purpose of the document is for a sponsor to fill out an affidavit of support form to sponsor an immigrant's application for permanent residency in the US.