Former President Carter said yesterday that President Clinton should designate the coastal plain of the Arctic National Wildlife Refuge a national monument “without further delay,” shielding it from the threat of oil and gas drilling. Carter said he would personally talk with Clinton about protecting the 1.5 million-acre coastline in Alaska, and he urged environmentalists to write Clinton about the issue. Spokespeople from both the White House and Interior Department said additional protections for the coastline were not being considered. Carter was in Alaska to celebrate the 20th anniversary of the Alaska National Interest Lands Conservation Act, which he signed to put more than 100 million acres of federal land into parks, refuges, and other reserves.