Social Security is expected to reach insolvency within a decade, and with the key safety net program facing the prospect of automatic benefit cuts when that occurs, a critical debate over the cost of replacing that revenue could be obscured by a budget rule.

Kent Smetters, faculty director of the Penn Wharton Budget Model, spoke exclusively with FOX Business about a new report that details how federal budget rules assume Social Security benefits would be paid in full using general revenue transfers when the program's trust funds reach insolvency in about six or seven years.

"What the law actually says is that when the trust fund depletes, benefits have to be reduced to a level that's consistent with just the monthly payroll taxes that come in.…