"This election and every election is about who will have the power to shape our children for the next four or eight years of their lives," Mrs. Obama said on the opening night of the Democratic convention.
"There is only one person I trust with that responsibility, only one person I believe is truly qualified to be president of the United States, and that is Hillary Clinton."
VIDEO: Watch Michelle Obama's entire speech
The first lady was among a high-wattage line-up of speakers taking the stage, all but wiping away earlier tumult that had exposed deep tensions between Clinton supporters and those loyal to her primary opponent Bernie Sanders.
While Mrs. Obama has often avoided overt politics during her nearly eight years in the White House, her frustration with Trump's rise was evident. She warned that the White House couldn't be in the hands of someone with "a thin skin or a tendency to lash out" or someone who tells voters the country can be great again.
PHOTOS: The 2016 DNC in Philadelphia
"This right now, is the greatest country on earth," she said.
An array of office holders and celebrities hammered home the call for unity, with singer Paul Simon singing his "Bridge Over Troubled Water" as delegates linked arms and swayed to the music.
Former President Bill Clinton smiled and clapped from the audience.