This bill, titled the "Medicare and Social Security Fair Share Act," seeks to bolster the financial solvency of the Social Security and Medicare trust funds. It proposes significant amendments to the Internal Revenue Code of 1986, primarily by modifying payroll taxes and expanding the tax on unearned income for high-income individuals. These changes are designed to increase revenue streams for both programs. Specifically, the bill modifies the wage base for Social Security (Old-Age, Survivors, and Disability Insurance) taxes. It would apply the Social Security payroll tax to wages and self-employment income exceeding $400,000, while maintaining the current wage cap for earnings below it, effectively creating a gap in taxation for earnings between the current cap and $400,000. Additionally, it introduces a new, additional 1.2% Hospital Insurance (Medicare) tax on wages and self-employment income. This extra Medicare tax would apply to earnings above $500,000 for joint filers, $250,000 for married individuals filing separately, and $400,000 for all other taxpayers. The legislation also significantly expands the Net Investment Income Tax (NIIT) for high-income individuals, trusts, and estates. For individuals with modified adjusted gross income exceeding $500,000 (joint) or $400,000 (other), the definition of income subject to NIIT is broadened to include certain active business income , termed "specified net income." The bill increases the NIIT rate for these high-income taxpayers, as well as for trusts and estates, from the current 3.8% to a new rate of 17.4% . This expanded tax also includes certain foreign income and clarifies that net operating losses are not considered. Crucially, the bill mandates that a portion of the revenue generated from the expanded Net Investment Income Tax be transferred directly to the Social Security and Medicare Trust Funds . Specifically, 71.3% of these taxes would go to the Old-Age and Survivors Insurance Trust Fund, 10.3% to the Disability Insurance Trust Fund, and 28.7% to the Federal Hospital Insurance Trust Fund. The payroll tax amendments would take effect on January 1 of the first calendar year after enactment, while the unearned income tax changes would apply to taxable years beginning after December 31, 2025.