The "Medicare and Social Security Fair Share Act" proposes significant changes to the Internal Revenue Code of 1986 to bolster the financial stability of Social Security and Medicare. A core provision involves modifying payroll taxes by removing the existing cap on earnings subject to Social Security taxes for income exceeding $400,000. This creates a "donut hole" where income between the current Social Security wage base and $400,000 remains untaxed, but income above $400,000 becomes subject to the tax. Additionally, the bill introduces a further additional Hospital Insurance tax of 1.2 percent on wages and self-employment income. This new tax applies to earnings above $400,000 for individuals, $500,000 for joint filers, and $250,000 for married individuals filing separately. These changes to payroll and self-employment taxes are slated to take effect on or after January 1 of the first calendar year following the act's enactment. The legislation also targets unearned income by substantially modifying the Net Investment Income Tax (NIIT). For high-income individuals, defined by a modified adjusted gross income exceeding $400,000 (or $500,000 for joint filers), the NIIT base is expanded to include a broader category of "specified net income," removing certain exclusions for business-related income. Furthermore, the bill imposes an additional 13.6 percent tax on this expanded investment income for high earners, effectively raising the total NIIT rate from 3.8 percent to 17.4 percent on applicable income. For trusts and estates, the NIIT rate is directly increased from 3.8 percent to 17.4 percent, applying to undistributed specified net income or net investment income. A crucial aspect of these unearned income tax modifications is the explicit allocation of the generated revenue. Specific percentages of the taxes imposed under Section 1411 of the Internal Revenue Code will be transferred to the Federal Old-Age and Survivors Insurance Trust Fund (71.3 percent), the Federal Disability Insurance Trust Fund (10.3 percent), and the Federal Hospital Insurance Trust Fund (28.7 percent), with these provisions taking effect for taxable years beginning after December 31, 2025.