ATLANTA — At a protest on the steps of the Georgia Capitol in 1992, Stacey Abrams, now the Democratic candidate for governor, joined in the burning of the state flag, which at the time incorporated the Confederate battle flag design and was viewed by many as a lingering symbol of white supremacy. Ms. Abrams’s role in the protest, which took place around the end of her freshman year at Spelman College in Atlanta, has begun to emerge on social media on the eve of her first debate Tuesday with her Republican opponent, Secretary of State Brian Kemp. Mr. Kemp and...