Former President Barack Obama sought to transfer the energy of his political movement to Vice President Kamala Harris at a rally on Thursday night outside Atlanta — their first joint appearance of the campaign — as he tried to help propel her over the finish line.

“Together, we have a chance to choose a new generation of leadership in this country,” Mr. Obama told a crowd of 23,000 people at a high school football stadium in Clarkston, Ga. “And start building a better and stronger and fairer and more hopeful America.”

When Ms. Harris took the stage, he lifted up her arm like a prizefighter in celebration. She quickly seemed to try to adopt his mantle, leading the audience, the largest she has drawn since becoming the Democratic nominee, in a chant of “Yes, we can,” Mr. Obama’s 2008 campaign slogan.