United States President Barack Obama addresses the delegates on the third day of the Democratic National Convention at the Wells Fargo Center in Philadelphia, Pennsylvania, USA, 26 July 2016.

President Barack Obama said Wednesday that the America he knows is "full of optimism" in an outright rejection of Republican presidential nominee Donald Trump's portrayal of a fearful and pessimistic country.

Speaking at the Democratic National Convention, Obama slammed Trump, saying he is "just offering slogans" and fear.

"He’s betting that if he scares enough people, he might score just enough votes to win this election," Obama said.

Contrasting Trump's "deeply pessimistic" vision of the United States, Obama said, "The America I know is full of courage and optimistic ... and decent and generous."

Obama made the last of four major speeches Wednesday on Clinton's behalf at the convention in Philadelphia, which goes into its fourth and final day on Thursday.

In addition to Obama, the Clinton campaign deployed Vice President Joe Biden, Clinton's vice pesidential candidiate Tim Kaine and former New York mayor Michael Bloomberg.

Obama had already backed Clinton, but on Wednesday before thousands of delegates and guests, Obama put his formidable campaigning skills to work for her. He could prove to be her most important so-called "uber" and an asset few presidential candidates have.

Obama, who is wildly popular among Democrats, extolled Clinton as someone supremely qualified to be president.

“I can say with confidence there has never been a man or a woman, not me, not Bill, more qualified than Hillary Clinton to serve as President of the United States of America,” Obama said.

He also said she has the judgment, experience and the temperament to be the next commander-in-chief and continue the fight against terrorism.

"I know Hillary won’t relent until ISIL is destroyed," he says, adding that she will do it without resorting to torture, or banning Muslims from entering the country.

He said she knows she’s made mistakes, but "that’s what happens when we try."

Clinton appeared with Obama on stage after he finished speaking. They hugged and exchanged a few words before turning to the crowd, smiling and waving at the delegates.

Obama said unlike the portrait Trump has painted of the state America is in as he finishes his second term, he believes America is full of courage, optimism and ingenuity and is decent and generous.

Obama acknowledged there is still work to do, but urged Americans to "reject cynicism and fear" being projected by the Republican party. The 2016 presidential election represents a "fundamental choice about who we are as a people," he said, adding that electing Clinton would "show the world we still believe in the promise of this great nation."

Obama has delivered major speeches at every party convention since 2004, the year that as a state senator from Illinois he was first noticed as a rising star in the party. Four years later he was chosen as the party' presidential nominee and won the White House.

Clinton continued to tap into her deep bench of supporters as the convention builds toward her acceptance of the nomination on Thursday night.

The evening's speeches also gave the party the chance to introduce Kaine, largely a political unknown in the US, in a nationally televised speech.

He formally accepted the vice presidential nomination and spoke about his career in politics and civil rights law. Kaine called Trump a "one-man wrecking crew."

"We'd better elect the candidate who's proven she can be trusted with the job ... who's proven she's ready for the job," Kaine added.

"Hillary [Clinton] is ready to fight, she's ready to win," Kaine said.

Kaine also delivered remarks in Spanish, drawing cheers from party delegates.

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