Michelle Obama has told a diverse and enthusiastic crowd in Philadelphia that candidates do not change once they become president and that America "needs an adult in the White House".
The First Lady never mentioned Republican presidential nominee Donald Trump during the rally at LaSalle University.
However, there was no question that she was warning voters that candidate Trump would be the same as President Trump.
"The presidency doesn't change who you are, it reveals who you are," she told the cheering crowd.
She referred to several comments Mr Trump made during and after Monday's debate, including his apparent acknowledgement that he has paid no taxes in some years.
Mr Trump said that makes him "smart".
"If a candidate is erratic and threatening, if a candidate traffics in prejudice, fear and lies on the campaign trail, if a candidate thinks not paying taxes makes you smart, or that it's good business when people lose their homes; if a candidate regularly and flippantly makes cruel and insulting comments about women, about how we look, how we act - well, sadly, that's who that candidate really is," she said.
"That is the kind of president they will be."
She said the country needs a leader who is steady and measured because when making war-or-peace decisions, "the president can't just pop off or lash out irrationally. No, we need an adult in the White House, I guarantee you."
She cast Hillary Clinton as a tough, compassionate fighter who does not back down and who loves her country.
"Experience matters, preparation matters, temperament matters," she said.
"Hillary Clinton has it all. She's the real deal."
Mrs Obama was heading to Pittsburgh for another rally for the Democratic presidential nominee later on Wednesday.
Mr Trump's campaign reacted to Mrs Obama's speech with a statement saying Mrs Clinton is in "panic mode" in Pennsylvania because polls show the Republican presidential nominee surging in the battleground state.