By Valerie Volcovici WASHINGTON, Feb 21 (Reuters) - Gina McCarthy, said to be U.S. President Barack Obama's choice as the nation's top environmental regulator, avoided questions about a potential promotion on Thursday but said states will play a key role in shaping federal regulations at an energy and climate policy forum. McCarthy, assistant administrator for the U.S. Environmental Protection Agency's Office of Air and Radiation since 2009, was tightlipped about whether Obama will nominate her as the EPA's next administrator.