With five weeks remaining before the Lebanese parliament must choose a new president, rightist Phalange leader Bashir Gemayel is still the only declared candidate. Phalange officials insist that elections must be held. Otherwise, says one, ''It will be the end of the republic, everyone will try to grab a piece of Lebanon.''
At this stage, one of the most talked-about compromise candidates, former President Camille Chamoun, is supporting Mr. Gemayel. Elections must occur by Sept. 13 and the successor to President Elias Sarkis must take office by Sept. 23.