In a big victory for environmentalists, the Clinton administration rejected a proposal yesterday to convert a former U.S. Air Force base near the Florida Everglades into a commercial airport. Enviros argued that an airport at the Homestead base would create significant pollution and threaten the ecosystems of two nearby parks, the Everglades, 10 miles away, and Biscayne National Park, two miles away. After years of study, the administration agreed and suggested that Miami-Dade County officials pursue a more limited development plan. Homestead Air Base Developers, the group proposing to develop the airport, said it would fight the decision in federal court. During the presidential race, Vice President Al Gore refused to come out against the airport plan, causing some enviros in Florida to support Green Party candidate Ralph Nader.