Medicare and Social Security Fair Share Act

Creates new higher-rate payroll and investment tax rules aimed at high earners and sets how those taxes are collected and credited. It adds a $400,000 Social Security taxable wage band, imposes an additional Medicare (hospital-insurance) surtax on very high wages and self-employment income, and raises/expands taxes on certain investment income, including for trusts and estates, with portions of the revenue directed to Social Security and Medicare trust funds.

Imposes new withholding and collection rules for employers and successor employers, adjusts railroad retirement tax treatment to match the new wage band, prevents a deduction for the new self‑employment tax, and tasks Treasury with issuing regulations on certain foreign-corporation income treatments. Effective dates are set relative to enactment as specified in the text.