HOUSE G.O.P. BACKS BUSH AND PURSUES A COMMON AGENDA
House Republican leaders are so united behind Gov George W Bush of Texas for their party's Presidential nomination that they have already begun meeting informally with him to discuss a common agenda for 2000 elections; these meetings are latest demonstration of extraordinary degree to which party apparatus has concluded that Bush's nomination is inevitable, even though there are nearly dozen contenders; Bush has yet to make a formal announcement of his candidacy, and first primary votes will not be cast for nine months; Bush photo (M)