Former Secretary of State Hillary Clinton secured the Democratic Party's 2016 nomination for the White House on Tuesday, becoming the first woman to head the ticket of a major party in US history.

In a symbolic show of party unity, Clinton's former rival, US Senator Bernie Sanders of Vermont, told the chairwoman from the convention floor that Clinton should be selected as the party's nominee during a state-by-state roll call at the Democratic convention in Philadelphia.

Earlier, delegates from South Dakota had given Clinton 15 votes, ensuring that she had more than the 2,383 votes needed to win the nomination. She emerged with a total of 2,842 votes to Sanders' 1,865 votes.

After a tough battle with Sanders, Clinton is now the party's standard-bearer against Republican nominee Donald Trump in the Nov. 8 election.

Delegates chanted "Hillary, Hillary" as US Senator Barbara Mikulski of Maryland formally put forward Clinton's name for the alphabetical roll-call vote.

"Yes, we do break barriers, I broke a barrier when I became the first Democratic woman elected to the Senate in her own right," Mikulski said. "So it is with a full heart that I'm here today to nominate Hillary Clinton to be the first woman president," Mikulski said.

Sanders has endorsed Clinton, a former first lady and US senator, but some of his supporters protested in Philadelphia against the party leadership's apparent backing of her during the bitter Democratic primary fight.

Supporters of Clinton say her Washington credentials show she has the experience needed for the White House during troubled times as the United States tries to speed up its economic recovery and faces security challenges abroad.

Detractors view her as too cozy with the establishment and say she carries political baggage dating back to the start of her husband President Bill Clinton's first White House term in the 1990s.

Clinton had been leading Trump in national opinion polls in recent weeks but the New York businessman got a boost from the Republican National Convention in Cleveland where he was formally nominated last week.

Trump had a 2-point lead over Clinton in a Reuters/Ipsos opinion poll released on Tuesday, the first time he has been ahead since early May.

Clinton, who promises to tackle income inequality and rein in Wall Street if she becomes president, is eager to portray former reality TV star Trump as too unstable to sit in the Oval Office. – Reuters