Former President Bill Clinton speaks on the second night of the Democratic National Convention in Philadelphia, Pennsylvania, US, July 26, 2016. [Photo/Agencies]

PHILADELPHIA - Former President Bill Clinton portrayed his wife Hillary on Tuesday as a force for change and a longtime fighter for social justice as he made a case for her historic 2016 bid for the White House.

The ex-president told the Democratic Party convention in Philadelphia that Hillary Clinton was "the best darn change-maker I've ever met in my entire life."

"If you were sitting where I am sitting and you have heard what I have heard at every dinner conversation, every lunch conversation, every long walk, you would say this woman has never been satisfied with the status quo in anything. She always wants to move the ball forward, that is just who she is," he said.

Earlier on Tuesday, Hillary Clinton secured the Democratic Party's 2016 nomination for the Nov 8 election, becoming the first woman to head the ticket of a major party in US history.

Bill Clinton told the convention in a keynote speech that Hillary had been an activist for social justice since the couple's early days as law students together. He told how she gave legal aid services to poor people and went undercover to expose a segregationist school in Alabama in the 1970s.

"She's insatiably curious, she's a natural leader," he said, describing her as the Clinton family's "designated worrier" who was "born with an extra responsibility gene."

After a tough battle with US Senator Bernie Sanders during the state-by-state nominating contests, Clinton is now the party's standard-bearer against Republican nominee Donald Trump.

President from 1993 to 2001, Bill Clinton, 69, left office with high approval ratings and is known as one of the most powerful political orators in the country.

His speech offered an unusual twist to the warm spousal endorsement of a presidential candidate traditionally given in party conventions by a wife, not a man - let alone a former president of the United States.

Hillary Clinton's nomination was a milestone in America's 240-year-old history. US women got the right to vote in 1920 after ratification of the 19th Amendment to the Constitution.