The President's Twitter posts come after Obama delivered a slapdown of Trump, saying he is the "wrong President" for the United States, and is "clearly in over his head".

In a clear reference to Trump's viral Axios interview at the start of this month, where he downplayed the US Covid-19 death toll, Obama appeared to use the President's own words to say he is not up to governing: "It is what it is."

Obama's words were uncharacteristically pointed, and she referenced her own phrase from four years ago at the last Democratic convention: "When they go low, we go high."

"When they go lie, we go high. Going high is the only thing that works," Obama said.

"But let's be clear. Going high does not mean putting on a smile and saying nice things when confronted by viciousness and cruelty. Going high means taking the harder path. It means scraping and clawing our way to that mountaintop.

"Going high means unlocking the shackles of lies and mistrust with the only thing that can truly set us free – the cold, hard truth. So let me be as honest and clear as I possibly can.

"Donald Trump is the wrong President for our country. He has had more than enough time to prove that he can do the job, but he is clearly in over his head.

"He cannot meet this moment. He simply cannot be who we need him to be for us. It is what it is."

Obama's comments were in reference to Trump's downplaying of the surging coronavirus death toll during an interview with Australian reporter Jonathan Swan earlier this month.

During the now-infamous interview, Swan asked the President about the government's coronavirus response — and the President responded, "It is what it is".

"I've gone to your rallies. I've talked to your people. They love you. They listen to you. They listen to every word you say. They hang on your every word," Swan said to the President.

"And so when they hear you say, 'everything's under control. Don't worry about wearing masks,' I mean, these are people — many of them are older people."

Trump responded: "Well what's your definition of control? I think it's under control."

"How?" Swan responded. "A thousand Americans are dying a day."

"They are dying. That's true. And you, it is what it is," Trump said. "But that doesn't mean we aren't doing everything we can. It's under control as much as you can control it."

The Former First Lady was speaking at the Democratic Party's National Convention, a four-night event which will end with Joe Biden formally accepting the nomination as the party's presidential candidate.

"Now I understand that my message won't be heard by some people," Obama said. "We live in a nation that is deeply divided and I am a black woman speaking at the Democratic Convention.

"But enough of you know by me now. You know that I tell you exactly what I'm feeling. You know that I hate politics. But you also know that I care about this nation.

"You know how much I care about all of our children. So if you take one thing from tonight let it be this — if you think things cannot possibly get worse, trust me, they can and they will, if we don't make a change this election."