Four taps of the microphone were all Michelle Obama needed to take down Donald J. Trump.

“Hillary Clinton is tough,” Mrs. Obama, the first lady, told a crowd of several thousand in Charlotte, N.C., on Tuesday. “See,” she continued, “I’ve watched her when she gets knocked down — she doesn’t complain. She doesn’t cry foul.”

Tap, tap, tap, tap.

It took a second for the audience to catch on: Mrs. Obama was mocking Mr. Trump’s complaint that a faulty microphone had hindered his performance in his first presidential debate with Mrs. Clinton. But as Mrs. Obama continued — “No, she gets right back up, comes back stronger” — the mostly young and heavily African-American crowd let out a deafening howl.

They called her “the closer” during Barack Obama’s 2008 campaign, like a go-to relief pitcher used sparingly but with devastating effect. Mrs. Obama preferred spending time with her daughters over attending political rallies. But when she was unleashed before a pivotal caucus or primary, her story of growing up on the South Side of Chicago and falling in love with a young community organizer had an unmatched, almost magical power to turn out voters, campaign officials discovered.

Eight years later, Mrs. Obama is playing a similar role for the woman whom she helped her husband defeat in that race.