Iowa’s only democrat Congressman joined a walk-out yesterday on the bill which establishes a new prescription drug benefit for the nation’s elderly. Under the bill, Medicare officials would merely oversee private insurance company prescription drug plans for senior citizens. Congressman Leonard Boswell, a democrat from Davis City, preferred the democrats’ alternative.Critics say the democrat plan was twice as expensive as the G-O-P proposal, but Boswell says the G-O-P plan is unworkable because insurance companies say they will not offer prescription drug coverage for the elderly.Congressman Greg Ganske, a republican from Des Moines, was one of 10 republicans to join Boswell and the other democrats in voting against the G-O-P prescription drug plan. Iowa’s other Republican Congressmen, Tom Latham, Jim Nussle and Jim Leach, all voted for it.