Social Security Expansion Act

Amends title II (Old Age, Survivors and Disability Insurance) of the Social Security Act to:

• increase the primary insurance amount for all eligible beneficiaries, beginning in 2021;
• revise computation of cost-of-living adjustments to use the Consumer Price Index for Elderly Consumers;
• increase the special minimum primary insurance amount for lifetime low earners based on years in the workforce.

Amends the Internal Revenue Code to: (1) apply employment and self-employment taxes to remuneration up to the contribution and benefit base and to remuneration in excess of $250,000; and (2) increase the tax on investment gain from 3.8% to 10% of the lesser of net investment income for such taxable year or the excess (if any) of the modified adjusted gross income for such taxable year, over the threshold amount, with 62% of such tax allocated to the Federal Old-Age and Survivors Insurance Trust Fund and 9% allocated to the Federal Disability Insurance Trust Fund.