US President Barack Obama will spearhead a Democratic effort to paint Donald Trump as unfit to be commander-in-chief today, hoping to capitalise on the mogul's missteps in an escalating scandal about alleged Russian spying.
Trump courted outrage as Democrats gathered in Philadelphia for day three of their national convention by encouraging Moscow to hack the email of his election rival Hillary Clinton.
Obama will be one of a string of Democratic national security heavyweights -- from former CIA director Leon Panetta to Vice President Joe Biden -- who will appear before the convention to explicitly and implicitly question Trump's temperament.
In an interview, Obama warned that "anything is possible" when it came to November's election.
"What I think is scary is a president who doesn't know their stuff and doesn't seem to have an interest in learning what they don't know," he said, referring to Trump.
Obama, acting as a character reference for his former primary foe and secretary of state, will also make the case that Clinton is uniquely qualified to be president.
"I can say with confidence there has never been a man or a woman more qualified than Hillary Clinton to serve as President of the United States of America," Obama will say, according to released excerpts of his speech.
"You know, nothing truly prepares you for the demands of the Oval Office. Until you've sat at that desk, you don't know what it's like to manage a global crisis or send young people to war," he will say.
"But Hillary's been in the room; she's been part of those decisions."
In an incendiary press conference, Trump launched a pre-emptive attack against Obama, calling him "the most ignorant president in our history."
The Republican nominee also caused howls of outrage from Democrats, with a reference to thousands of Clinton emails held on a private server that were deleted.
"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing." Trump said.
