PHILADELPHIA (REUTERS) - US President Barack Obama will add an optimistic pitch to the campaign to elect Hillary Clinton as the first woman US president, as he seeks to hand off the White House to a trusted fellow Democrat and stop Republican Donald Trump.

Clinton formally secured the Democratic Party's presidential nomination on Tuesday, coming back from a stinging defeat to Obama in her first White House run in 2008 and surviving a bitter primary fight against Bernie Sanders to become the first woman to head the ticket of a major US party.

The 68-year-old former secretary of state will accept the nomination on the last day of the party's convention in Philadelphia on Thursday, becoming the Democratic standard-bearer against Trump in the Nov 8 election.

Obama, scheduled to address the meeting on Wednesday evening US time (Thursday morning Singapore time), has been vocally critical of the Republican candidate, and is likely to contrast his optimistic view of the United States with Trump's darker vision.

The New York businessman has cast America as a place in need of a strong leader, where security threats abound and law and order are breaking down. Trump, 70, has proposed deeply controversial measures such as temporarily banning Muslims from entering the country and building a wall on the Mexico border to stop illegal immigrants.

"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 per cent convinced that Hillary Clinton can be a great president," Obama said in an interview with NBC News that aired on Wednesday.

His remarks will follow his wife Michelle Obama's opening night speech to the gathering on Monday, which was a rousing success with delegates.

"I'm not going to hit that bar, so let me concede top speech-making already to my wife, but I couldn't have been prouder of her," Obama 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.

But Obama's speech will 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 a US senator to the Democratic convention in 2004, which effectively 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 Sanders, a US senator from Vermont.

The Clinton campaign portrays Trump, a former reality TV star whose campaign style has been freewheeling and whose remarks have been peppered with insults, as temperamentally unfit for the White House. The convention aimed to reinforce this message on Wednesday, focusing on national security.

Campaign chair John Podesta said the day's events would seek to contrast Clinton's approach to national security with Trump's "unsteady, unfit and dangerous approach".

The Democrats have sought to buttress Clinton with a star gathering of current and past party notables in Philadelphia. As well as Obama, speakers on Wednesday will include Clinton's vice presidential running mate, Tim Kaine, Vice President Joe Biden and former New York Mayor Michael Bloomberg.

Bloomberg, founder of the Bloomberg news and data service, was previously elected as a Republican and later became an independent. The former mayor, who could help Clinton win over independent and moderate voters, considered running for president as an independent candidate earlier this year.

But he dropped the idea in March and made plain his reason: He did not want to increase the chances that the then front-runners in the Republican primary race, Trump and US Senator Ted Cruz of Texas, could end up in the White House.

Also speaking will be vice presidential candidate Kaine. Announcing her choice of running mate last week, Clinton cast the US senator from Virginia as a seasoned and steady leader and a contrast with Trump's volatile campaign.

But the pick disappointed some supporters of Sanders who were upset over Kaine's past support for the Trans-Pacific Partnership trade agreement.

On Tuesday night, Democrats reveled in history being made. "If there are any little girls out there who stayed up late to watch, let me just say: I may become the first woman president, but one of you is next," Clinton told the convention via a video satellite link.

Addressing the gathering, former President Bill Clinton called his wife, who has also been a US senator, a dynamic force for change.

Bill Clinton also sought to paint his wife in a warm, personal light, countering the Republican portrayal of her as a power-hungry politician who bends the rules and lacks transparency in her political dealings.

Opinion polls show many Americans distrust Hillary Clinton.

Controversy over her use of a private email server for official business while she was secretary of state from 2009-2013 dogged her during the campaign, and more than half of American voters view her unfavorably, according to Reuters/Ipsos polling.

Trump, who has never held elective office, is even less popular, with 61 percent holding an unfavorable view, the polling showed.

Trump got a boost in opinion polls from his nomination at the Republican convention in Cleveland last week. He had a 2-percentage-point lead over Clinton in a Reuters/Ipsos poll released on Tuesday, the first time he has been ahead in this survey since early May.