ATLANTA — Johnny Isakson will compete for a third term in the U.S. Senate in November. The Georgia Republican easily won Tuesday’s primary over long-shot challengers Derrick Grayson and Mary Kay Baccalao, capturing more than three-quarters of the votes cast. Jim Barksdale, the Democrat favored by top party members, meanwhile, tried to prevent a July runoff in the bid to face Isakson in the fall.