Senators Bennet and Kaine’s Medicare-X Choice Act of 2021 proposal would phase in a public option plan into the nongroup market over four years and introduce the public option plan into the small-group market in the fourth year. Other key features include a national reinsurance program, negotiation of prescription drug prices for the public option and Medicare Part D, and the elimination of cost sharing for primary care services. We used the Urban Institute’s Health Insurance Policy Simulation Model, also called HIPSM, to estimate the effects on coverage and spending of Medicare-X’s major provisions. We find that the legislation would reduce the number of uninsured people by 1.1 million in 2023. If the public option plan were fully phased in that year, employers would spend $10.8 billion less on health care and households would save $10.9 billion. The federal government would save $20.3 billion in 2023 and $250 billion over 10 years. Most of the proposal’s effects are on regions with the highest premiums.