Jesse L. Jackson, who won the endorsement of the Alabama New South Coalition last month, yesterday won the support of that state's other powerful, predominantly black political organization, the Alabama Democratic Conference. But the ADC, which endorsed Walter F. Mondale in 1984, is hedging its bet. While the group is giving its unanimous endorsement to Jackson, it named Sen. Albert Gore Jr. (D-Tenn.) as an alternative.

Joe Reed, ADC political director, said blacks across the South need to be ready to unify behind another Democratic candidate if Jackson does not win the nomination.

Both Jackson and Gore met with group members yesterday before the endorsement vote. Both were unable to appear when the ADC convened Dec. 5 to endorse a presidential candidate, and the group withheld its endorsement. Reed said a caucus rule requires candidates to appear at the endorsement or screening session to get the group's nod.

The selection of an alternative candidate is the result of a rules change adopted in October. The new rule would allow conference members who are Democratic National Convention delegates to band together with other black delegates to pick an alternative candidate if Jackson's bid for the nomination fails. "We've got to go to the inevitable . . . to have a spare tire," Reed said.