Barely a year ago, House Democrats replaced ailing Appropriations Chairman Jamie L. Whitten (D-Miss.) with the equally elderly but more vigorous William H. Natcher (D-Ky.). But now the rapid failure of the 84-year-old Natcher's health has touched off a vigorous campaign for his job. The fight began when Wisconsin Democrat David R. Obey, 56, a ranking member of the appropriations panel, began campaigning openly for Natcher's post. Obey's outspoken liberalism and brusque manner have won him a slew of enemies, and other House members organized an "anyone but Obey" campaign behind the committee's third-ranking member, Neal Smith (D-Iowa), a 74-year-old moderate. But Natcher, who is suffering from both heart and intestinal ailments, is still hanging in. He holds a record of responding to 18,937 roll-call votes without an absence, though recently the House leadership has had to delay actions to accommodate his medical treatments.