AIDS activists around the country think they might have a shot at lifting a two decades-long ban on federal funding for needle exchange programs. Many of the nation's 200 such programs are facing severe budget cutbacks in their states. Several needle exchanges in California have recently been eliminated. In Maine, demand for clean needles has risen dramatically as injection drug use has grown. But none of Maine's four programs will qualify for the federal cash infusion if a particular amendment is approved by Congress.