We use the Urban Institute’s Health Insurance Policy Simulation Model to simulate health coverage under several policy scenarios in 2028. The model is calibrated based on state-specific Medicaid and Marketplace enrollment and costs for early 2026. There is uncertainty in our estimates of the impacts of OBBBA’s Marketplace provisions on enrollment. This is particularly true for changes in PTC eligibility by immigration status, where no public data on the detailed immigration status of Marketplace enrollees with PTCs is available. Coverage losses among lawfully present immigrants may be higher than we simulate. There is also uncertainty in how many people would re-enroll in the absence of automatic enrollment; in the share of people who, when faced with a data match issue (DMI), would pay the full premium until the DMI is resolved; and the extent to which DMIs will be triggered as they are now rather than as proposed by the Notice of Benefit and Payment Parameters issued by the Centers for Medicare & Medicaid Services. In each case, we use the best available evidence to predict behavior. However, our estimates could understate coverage losses depending on how policy changes are implemented, particularly in the first year in which automatic enrollment with advance PTCs is prohibited.