The Obama administration will next week unveil a cornerstone of its climate-change initiative with a proposed rule aimed at allowing states to use cap-and-trade systems, renewable energy and other measures to meet aggressive goals for reducing carbon emissions by existing power plants. Energy companies and others affected by the proposal will be watching for key details, including the percentage by which companies and states must reduce carbon emissions, which is expected to be proposed in a range instead of a single number. The baseline year against which those reductions are calculated will also be closely monitored. But the proposal is designed to give states, which will administer the regulations, flexibility to meet the benchmarks, as opposed to placing emissions limits on individual plants, according to people familiar with the Environmental Protection Agency’s work on the rule. Central to the strategy of flexibility: the option to include a cap-and-trade component […]