US President Barack Obama will spearhead a Democratic effort to paint Donald Trump as unfit to be commander-in-chief Wednesday, contrasting the Republican nominee with his more experienced rival Hillary Clinton.

Obama, acting as a character reference for his former primary foe and secretary of state, will make the case to Democrats gathered in Philadelphia for their national convention that Clinton is uniquely qualified to be president.

"Nothing truly prepares you for the demands of the Oval Office," the two-term president will say, according to excerpts of his speech released by the White House.

"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."

"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."

Having fought his own bad-tempered primary against Clinton eight years ago, Obama could offer potent testimony about how a rival became an ally and trusted advisor.

For four years, Clinton traveled the globe pushing Obama's foreign policies as America's top diplomat.


Framing the battle ahead

The White House would like to make November's crunch election a battle about competence versus incompetence, steady versus capricious, knowledge versus entertainment.

On Wednesday, Obama will be one of a string of Democratic national security heavyweights -- from former CIA director Leon Panetta to Vice President Joe Biden -- to appear before the convention to explicitly and implicitly question Trump's temperament.

"How can there be pleasure in saying 'You're fired'? He's trying to tell us he cares about the middle class? Give me a break! That's a bunch of malarkey!" a visibly angry Biden said.

In an incendiary press conference, Trump launched a pre-emptive attack against Obama, calling him "the most ignorant president in our history."

Democrats will hope to capitalize on a string of perceived foreign policy missteps, culminating in Trump's suggestion that Russia spy on his rival Clinton.

The Republican nominee caused howls of outrage Wednesday, with a reference to thousands of Clinton emails held on a private server that were deleted.


READ: Donald Trump encourages Russia to hack Hillary Clinton emails

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing." Trump said.

Clinton says the emails were personal and not work-related, but for Republicans, it is a smoking gun for a cover-up involving her use of a private server during her time at the State Department.

The FBI concluded this month that Clinton had been "extremely careless" in her handling of classified material via a private email server, but did not recommend that she face criminal charges.

"He asked the Russians to interfere in American politics. Think about that," Panetta said.

"As someone who was responsible for protecting our nation from cyber attacks, it is inconceivable to me that any presidential candidate would be that irresponsible."

Clinton on Tuesday became the first woman to win the presidential nomination of a major US political party, teeing up the November showdown with Trump.

Democrats had "put the biggest crack" yet in the glass ceiling for women, she said in a video message to the convention.


Dimming limelight

In the twilight of his second term, Obama faces ever-dwindling opportunities to address the nation, mold his legacy and influence the 2016 race.

But on Wednesday, he has a prime-time chance when he appears before thousands of delegates in Philadelphia and tens of millions of viewers at home.

The White House says Obama has been working on the roughly 30-minute speech for weeks.

Yet this touchstone presidential moment has been a decade or more in the making.

The address will bookend Obama's career-launching address to the Democratic convention in 2004, his contentious 2008 primary battle with Clinton and his eight years in office.

Aides said Obama will make a familiar case for what has been achieved during his two terms, highlighting America's recovery from the Great Recession.

Obama will also try to leverage his vast popularity among Democrats to unify a party scarred by the bruising primary campaign between Clinton and leftist Bernie Sanders.

The four-day confab in Philadelphia -- the City of Brotherly Love -- has so far been somewhat less than fraternal, with disappointed Sanders supporters periodically disrupting the proceedings with boos.