This paper examines electricity market responses to flexibility provisions in prospective renewable energy mandates and the geographical incidence of impacts. Using an integrated model of electric sector investments and operations with detailed spatial and temporal resolutions, the analysis demonstrates how renewable mandate trade formulations for electricity and renewable energy certificates can materially impact power sector outcomes like capacity planning decisions, compliance costs, CO2 emissions, and the regional distribution of renewable development. There are substantial welfare gains, up to $84 billion in present value terms through 2050, from inter-regional electricity and permit trade (and costs of market fragmentation), but the degree and direction of impact depend on region-specific considerations. Allowing permit trade encourages greater deployment of wind and solar in regions with favorable investment environments and resources, but renewable capacity additions are appreciable in all regions since diminishing marginal returns and transmission constraints limit the benefits of overdevelopment in any single region. Model results suggest that regions will likely find it beneficial to generate at least half of their renewable mandate compliance obligations through in-state resources and that most of the economic benefits from inter-regional REC exchange can be captured with a relatively modest amount of trading flexibility. Trade flexibility is shown to have minimal impacts on CO2 emissions leakage nationally.