Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support. In this post, we'll explore examples of proof of US domicile and the documents required to establish proof of domicile.This guide will clarify what domicile is, how to establish it, and the documentation you need for proof of domicile. 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. I'm at the stage of writing a letter of domicile as a supporting document to my I864. The sample letter I found online is very basic. 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).