Story highlights "A president can't just pop off or lash out irrationally," Obama said

Obama is Clinton's most popular campaign surrogate

New York (CNN) First Lady Michelle Obama tore into Donald Trump Wednesday, telling Hillary Clinton supporters, "We need an adult in the White House."

"When making life-or-death, war-or-peace decisions, a president can't just pop off or lash out irrationally," she said on behalf of the Democratic presidential nominee at La Salle University in Philadelphia. "We need an adult in the White House. I guarantee you."

Obama criticized Trump over his spreading of the birther conspiracy theory, continuing a rebuke she first made earlier this month while stumping for Clinton in Virginia.

"There are those who questioned and continue to question for the past eight years whether my husband was even born in this country," Obama said to boos. "And let me say: hurtful, deceitful questions deliberately designed to undermine his presidency. Questions that cannot be blamed on others or swept under the rug by an insincere sentence uttered at a press conference."

The first lady denounced the "negativity" and "name-calling" in the election and urged supporters to "choose the right person to lead our country forward."

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