A new climate change treaty must plug three gaps: the absence of emission targets extending far into the future, the absence of participation by the United States, China, and other developing countries, and the absence of reason to expect compliance. To be politically acceptable, it must obey certain constraints regarding country-by-country economic costs. We offer a framework to assign quantitative emission allocations, across countries, one budget period at a time. The two-part plan: (i) China and other developing countries accept targets at BAU in the coming budget period, the same period in which the US first agrees to cuts below BAU; (ii) all countries are asked in the future to make further cuts in accordance with a formula which sums a Progressive Reductions Factor, Latecomer Catch-up Factor, and Gradual Equalization Factor. An earlier proposal for specific parameter values in the formulas achieved the environmental goal that CO2 concentrations plateau at 500 ppm by 2100. It obeyed our political constraints: keeping the economic cost for every country below thresholds of Y=1% of income in Present Discounted Value, and X=5% of income in the worst period. In this paper we attain a concentration goal of 460 ppm CO2, but only by loosening political constraints.