Trump warns Michelle Obama made a ‘big mistake’ by being ‘nasty’ to him

‘She opened up a little bit of a box,’ the former president said, to jeers and laughter from his supporters

Donald Trump has ominously warned Michelle Obama that she made a "big mistake" by being "nasty" to him at a rally in Atlanta.

Speaking at the Georgia Institute of Technology on Monday night, the former president adopted a gleeful and ironical tone as he warned the former first lady that she had "opened up a little bit of a box".

"You know who was nasty to me? Michelle Obama," said Trump, prompting a loud chorus of boos, jeers, and shouts from his audience. "She was really – ooooooh."

"I always tried to be so nice and respectful," he went on, despite having spent years accusing the former first lady's husband of secretly being born in Kenya and therefore ineligible to be a US president.

"Oooooh! She opened up a little bit of a box. She opened up a little bit of something," said Trump. "She was nasty, whooo.

"Shouldn't be that way. That was a big mistake she made. With your support on November 5 we will achieve success that no one can imagine."

Michelle Obama welcomes Melania Trump and Donald Trump to the White House on the day of his inauguration, on January 20 2017 (Getty Images)

Trump's comments were a response to a speech by Michelle Obama in Michigan on Saturday, where she accused him of "gross incompetence", "erratic behavior", and "obvious mental decline".

“I hope you’ll forgive me if I’m a little frustrated that some of us are choosing to ignore Donald Trump’s gross incompetence, while asking Kamala to dazzle us at every turn,” said Obama.

“I hope that you’ll forgive me if I’m a little angry that we are indifferent to his erratic behavior, his obvious mental decline, his history as a convicted felon, a known slum lord, a predator found liable for sexual abuse.”

Those words were a reference to Trump’s guilty verdict in Manhattan in his hush money scandal, his infamous history as a landlord, and a verdict in a defamation trial from a woman who accused him of sexual assault.

Later in Trump's own speech on Monday, when he brought up Michelle Obama again, some of his supporters began chanting: "Lock her up! Lock her up!" Trump was silent for a few seconds, before admonishing them: "Be nice."

It came during a busy day of campaigning for both Trump and Kamala Harris, with the latter giving a rally in Ann Arbor, Michigan while Barack Obama and Bruce Springsteen fired up the crowd in Philadelphia, Pennsylvania.