Barack Obama will highlight Hillary Clinton's judgment and toughness as he seeks to boost her campaign to be the first female US president, hoping to hand off the White House to a trusted fellow Democrat and stop Republican Donald Trump.
On a day when Democrats meeting in Philadelphia planned to tout their candidate as far better suited than Trump to keep the country safe, the Republican on Wednesday gave his critics fresh fodder for attack, with remarks that the Clinton campaign said posed a possible national security threat.
Grabbing the spotlight at a news conference in Miami, Trump urged Russia to find and release tens of thousands of emails that Clinton did not hand over to US officials as part of a probe into her use of a private email system while she was secretary of state. Clinton has said the emails she did not hand over were private.
Wednesday's events at the party's national convention were aimed at contrasting the 68-year-old Clinton's foreign policy skills with Trump's "unsteady, unfit and dangerous approach," Clinton campaign chair John Podesta said earlier.
Clinton secured the party's nomination on Tuesday. When she formally accepts it on Thursday, she will become the Democratic standard-bearer against Trump in the November 8 election.
Trump, a 70-year-old New York businessman with no experience in political office, has hammered Clinton as untrustworthy and cast America as a place where security threats abound and law and order are breaking down.
He has proposed deeply controversial measures such as temporarily banning Muslims from entering the country and building a wall on the border with Mexico border to stop illegal immigrants.
The Clinton campaign portrays Trump, a former reality TV star, as temperamentally unfit for the White House.
Obama offered a preview of his evening speech in an interview with NBC News broadcast on Wednesday.
"I hope my headline (from the speech) is that the president of the United States is profoundly optimistic about America's future and is 100 percent convinced that Hillary Clinton can be a great president," he said.
Obama "has been candid about why he thinks electing the Republican nominee is a risky path for the United States," White House spokesman Eric Schultz said on Tuesday.
However, Obama's speech would focus more "on how Secretary Clinton has the judgment, the toughness and the intellect to succeed him in the Oval Office," Schultz said.
Obama, who beat Clinton in the 2008 campaign for the Democratic nomination, will be speaking 12 years to the day since he gave a keynote address, as an Illinois state senator, to the Democratic convention in 2004, which launched him on the national stage.
Clinton waged another hard-fought primary battle this year, beating off an unexpectedly strong challenge from the left by Bernie Sanders, a US senator from Vermont.
Democratic leaders have sought to tamp down lingering bitterness among some die-hard Sanders supporters, and move past unruly displays of dissent that marked the convention's first day on Monday.
In a gesture of party unity, Sanders put forward Clinton's name on Tuesday night to make her the first woman presidential nominee for a major US party.
Democrats have buttressed Clinton with a star gathering of current and past party notables. By contrast, many prominent Republicans were absent from the party convention that nominated Trump for the White House last week.
STAR LINE-UP
As well as Obama, speakers on Wednesday will include Clinton's vice presidential running mate, Tim Kaine; Vice President Joe Biden; former New York Mayor Michael Bloomberg and Leon Panetta, a former defence secretary and former CIA director.
Speaking on CNN on Wednesday, Panetta said Trump's remarks about Russia and the Clinton emails were "beyond the pale."
Bloomberg, founder of the Bloomberg news and data service, was previously elected as a Republican and later became an independent. He would take on Trump in the area where the New York real estate developer seeks to appeal to voters: his business acumen, said campaign chair Podesta.
"He (Bloomberg) will talk about the reason he has come to the conclusion that Hillary Clinton is the right choice to be a stable leader on economic matters, and why Donald Trump through his life in business is incapable of managing the economy, let alone his own affairs," Podesta said.
Kaine, a US senator from Virginia picked as Clinton's running mate last week, will also get a chance to present himself to the party and the country.
After being formally nominated on Tuesday night, Clinton got an enthusiastic endorsement from her husband, Bill Clinton. The former president called his wife, who has also been a US senator, a dynamic force for change.
He also sought to counter the image critics have of her as driven by political ambition, with anecdotes painting his wife in a warm personal light.
