Sen. Malcolm Wallop, Wyoming's senior Republican senator, has decided to bow out of politics after serving for 18 years in the U.S. Senate. A major player in federal water and energy policy, Wallop had considered a run for Wyoming governor, but decided instead that his "political Energizer bunny" had run out. He will return to his ranch near Big Horn, Wyo. Wallop, 61, first entered the Senate with the support of many Wyoming environmentalists but later was considered a foe of most mainstream environmental groups. He led Senate battles against grazing reform and has recently accused the Clinton administration of waging a "war on the West." Once he leaves the Senate, Wallop said, he may work to create a private Foundation for the West that would defend Western traditions much as the Sierra Club defends spotted owls. He said, "Some of us in the West feel like we're entitled to be called spotted owls and probably are more endangered than they are."