The 43,000-member Georgia Association of Educators on Friday announced it is supporting Barnes in November's election.
GAE President Calvine Rollins called Barnes "the best choice to be able to come in and hit the ground running" on problems like class size and education funding.
Teacher anger over Barnes' aggressive plans to overhaul the state's school system has been blamed for contributing to his 2002 re-election defeat to Sonny Perdue. The Georgia teachers' group declined to endorse a candidate in the 2002 race for governor, a move widely read as a rebuke to Barnes, who had secured their support in the 1998 general election.
Barnes has been courting teachers aggressively as part of his comeback bid and has apologized to educators.
"It was never my intent -- and it is not my intent now -- not to treasure teachers and learning," Barnes says in a video ad posted on his Web site and shipped to teachers.
In its release Friday, the GAE said Barnes "while still facing some backlash from his previous administration, now has the benefit of hindsight in what is truly needed to help our public schools and its students."
The campaign of opponent Nathan Deal said Friday said the endorsement "demonstrates how out of touch the union bosses are with Georgia's educators."
"Teachers haven't forgotten the arrogance and bullying of the Barnes administration, and most will vote for Nathan Deal," spokesman Brian Robinson said. He noted that in Barnes' home base of Cobb County, the association of educators has endorsed Deal.