Michelle Obama will make her first appearance with Hillary Clinton on the campaign trail on Thursday, attending a rally to try to encourage early voting in North Carolina.

Eight years ago, the thought of Mrs. Obama being Mrs. Clinton’s most powerful campaigner would have been unimaginable. The Democratic primary in 2008 between Mrs. Clinton and Barack Obama was grueling and brutal.

That Mrs. Obama has thrown herself into the role of defending Mrs. Clinton so completely and, in the minds of many political observers, so effectively is among the most notable story lines of the tumultuous 2016 campaign.