In a major policy shift, Salvadoran leftist rebels have offered to participate in and respect the outcome of national elections if the U.S.-backed government meets their conditions. The proposal from the guerrillas who have waged war against the government for nine years comes in a document, obtained by The Associated Press on Monday, signed by the five senior rebel leaders. Rebels of the Farabundo Marti National Liberation Front have contended that all five elections held this decade were maneuvers of the "U.S.-imposed counterinsurgency project."