Battling Southern Baptists headed toward a showdown decision that would either curb rule of their 14.6-million member denomination by fundamentalists, or perpetuate it for the next generation. More than at any time since the fundamentalist tide began rolling nine years ago, moderates seemed to feel they had an even chance of breaking it at this year's convention, which meets today. ``They have a chance because they have a good candidate,'' said Toby A. Druin, of Dallas, assistant editor of the weekly Baptist Standard, referring to the prospective moderate nominee for the denominational presidency. The Rev. Richard Jackson, the moderate from Phoenix, Ariz., is expected to be pitted against fundamentalist candidate Jerry Vines, a minister from Jacksonville, Fla. The nominations and voting for president of the denomination, the nation's largest Protestant body, come this afternoon. ``It's going to be neck and neck,'' said the Rev. Albert McClellan of Nashville, Tenn., retired long-time denominational executive. Through successive presidencies, whose appointments gradually determine ruling trustees of the denomination's far-flung operational network, fundamentalists nearly have completed their takeover. Their timetable says one more year would seal the process. But moderates see signs that many who have supported fundamentalists before think they have gone to excess in newly restrictive measures on institutions and agencies, driving out some widely respected personnel. Indications also have arisen of links between fundamentalist strategists and the religious right on social-political issues. That gives the conflict a broad political tinge rather than it being an intramural difference over Bible interpretation, as it often has been pictured. ``People are becoming alarmed,'' McClellan said. ``There's a definite swing against these extremes.'' However, some peculiar back-stage maneuvering has gone on that possibly could scuttle the expected contest between Jackson and Vines. The Rev. John Bisagno of Houston suggested that to dampen the head-on conflict, only one of the two be nominated as a consensus candidate this year, the other two years hence. That seemed unlikely, but the possibility was discussed that Bisagno himself might be nominated, which probably would reduce chances of the moderate-backed Jackson. A hint of fundamentalist strength also came in a preconvention pastors conference Monday afternoon when a fundamentalist, the Rev. Ralph Smith of Austin, was elected president. He defeated a moderate, the Rev. Paul Powell of Tyler. The presidency of the pastor's conference has been a stepping stone to the convention presidency for the last decade. Among the fundamentalist engineered steps that have intensified controversy during the last year: _ Endorsement of Judge Robert Bork for the U.S. Supreme Court, contrary to long-time denominational policy against siding for or against specific candidates. The conservative Bork later withdrew from the nomination. _ Adopting mission board policies against women clergy and divorced ministers in most cases, and instituting screening of personnel to assure that they took a literalist view of the Bible. _ Moves that drove from office the Rev. Randall Lolley as president of Southeastern Baptist Theological Seminary in Wake Forest, N.C., and the Rev. Larry Baker as head of the denomination's ethics agency, the Christian Life Commission. _ Ouster of the Rev. George Sheridan as head of a regional New Jersey interfaith unit because he contended that God's covenant with Jews remained valid. _ Pushing right-wing causes for capital punishment, prayer in public schools, aid to religious schools, and against all abortions except to save the mother's life.