For most green card based petitions, the Petitioner (i.e. U.S. citizen or U.S. employer) must file an Affidavit of Support as a sponsor to the beneficiary (intending immigrant). The Affidavit of Support is a contract between the sponsor and federal government and is enforceable. Should the intending immigrant become a Public Charge (i.e. receive public assistance, including food stamps, Medicaid, etc.), then the government can come after the sponsor to reimburse it for those expenditures. The Sponsor has a contractual obligation to maintain the beneficiary at a certain percentage above the Federal Poverty line (usually 125% above the poverty line; 100% the poverty line for active duty military).
The Affidavit of Support process has three steps. First, the sponsor must demonstrate that he or she has sufficient household income. Second, the sponsor can use certain assets belonging to the sponsor, the beneficiary, or other household members to reach the required poverty line. Third, the sponsor may secure a joint sponsor to help reach the required poverty line.
USCIS will use the Affidavit of Support and accompanying evidence to determine whether they believe the beneficiary will become a Public Charge. USCIS will consider many factors including current job status, earning from current and past employment, and current use of welfare benefits by the sponsor or the sponsor’s family members. An Affidavit of Support can be rejected even if it establishes an income over the required poverty line if USCIS believes the person will not be able to maintain the income.
The poverty guidelines, affected by household size can be found at the USCIS website here. Calculation and proving sufficient income can be a complex task for those that are hovering around the required poverty line or those that have had a recent negative event occur related to their income (i.e. loss of job or reduction in income).