A public option would let consumers buy a government-run plan (often modeled on Medicare or Medicaid) on the ACA marketplace. Different design questions affect impact: who is eligible, what providers are required to participate, and at what reimbursement rates.
The original ACA dropped its public option in 2009-10 negotiations. State-level versions (Washington's Cascade Care, Colorado's Colorado Option) have piloted designs.
Defenders argue a public option provides cost-disciplined coverage in markets with weak competition. Critics argue it could crowd out private plans (especially if it can pay Medicare rates), reduce provider participation, and become a political backdoor to single-payer.