Exelon, the electric company based in Chicago, will promise on Tuesday to cut greenhouse gas emissions by 2020 by an amount larger than its total emissions in 2008, in a bid to shape the debate on carbon dioxide rules and to get a jump on compliance. Many academic researchers and nonprofit groups have made proposals for cutting emissions, but Exelon's will be an unusual public presentation devised by a company that hopes to make money in the process. The plan relies heavily on conservation and having existing nuclear plants produce more power, but it includes smaller contributions from wind and sun energy. The reductions in greenhouse gas emissions will come by making Exelon's operations more efficient, cutting the energy use of its electricity customers and building low-carbon generators that would displace older, less-efficient plants, many operated by rivals, the company said. One reason for the pledge is to seek credit for actions that cut emissions of other companies, said Exelon's chairman and chief executive, John W. Rowe. For example, Exelon plans to help the factories that it serves do the same work with less electricity so that some generating stations, owned by Exelon or others, will burn less fuel. Exelon also wants to build generating stations that use natural gas more efficiently to replace coal plants in the Midwest and East