PHILADELPHIA/WASHINGTON: President Barack Obama will paint an optimistic picture of America's future and give full-hearted support to Hillary Clinton in a speech on Wednesday to help her become the first woman elected US president and stop Republican Donald Trump.

On a day when Democrats meeting at their convention in Philadelphia planned to tout their candidate as far better suited than Trump to keep the country safe, the Republican 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 those emails were private.

Wednesday's events in Philadelphia 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.

Clinton made history when she secured the party's nomination on Tuesday. When she formally accepts it on Thursday, she will become the Democratic standard-bearer against Republican nominee Trump in the Nov. 8 election.

"Only one candidate has the experience and judgement to be commander in chief and that's Hillary Clinton," said former CIA Director Leon Panetta.

Liberal delegates chanted: "No more war," as Panetta spoke. Others in the Wells Fargo Centre convention hall shouted: "USA, USA," in support of Panetta, also a former defence secretary.

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 to stop illegal immigrants.

The Clinton campaign portrays Trump, a former reality TV

star, as temperamentally unfit for the White House.

'100 PERCENT CONVINCED'

Obama will offer an alternative to Trump's vision of the United States as being under siege from illegal immigrants and terrorism and losing its way in the world.

“The America I know is full of courage, and optimism, and ingenuity. The America I know is decent and generous," Obama will say, according to excerpts of his speech released by the White House.

Nodding to voters' concerns, he will say he understands

frustrations "with political gridlock, worry about racial divisions" and Islamic State-inspired attacks in Orlando, Florida, and Nice, France.

"There are pockets of America that never recovered from factory closures; men who took pride in hard work and providing for their families who now feel forgotten," Obama will say.

He told NBC News his speech would make clear he is "100 percent convinced that Hillary Clinton can be a great president."

Obama's speech will focus "on how Secretary Clinton has the judgement, the toughness and the intellect to succeed him in the Oval Office," White House spokesman Eric Schultz said.

Obama, who beat Clinton in the 2008 campaign for the Democratic nomination, will be speaking 12 years to the day that he gave a keynote address, as an Illinois state senator, to the Democratic convention in 2004, which launched him on the national stage.

Obama, whose wife, Michelle, gave Clinton a rousing tribute at the convention on Monday, is scheduled to deliver his 30-minute speech at 10:30 p.m. EDT (0230 GMT on Thursday).

Free trade, an area where Clinton and Obama disagree, will not come up. National security will be featured but not emphasized.

One official said the president stayed up until 3:30 a.m. on Tuesday working on his remarks after watching the first lady's speech, suggesting some competition between the spouses after her well-received address.

"Like any successful marriage, they make each other better," the official said.

STAR GATHERING

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.

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 nominated for president by a major U.S. party.

Other 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 CIA director.

Delegates at the convention formally nominated Kaine as the party's vice presidential candidate by voice vote on Wednesday.

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.

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.