Hillary Clinton to run for United States president in 2016 race.

Hillary Clinton promised to be a champion for everyday Americans as she kicked off a long-awaited second run for the White House as the commanding Democratic frontrunner.

Ms Clinton, 67, made the announcement in a video released online, ending prolonged speculation she once again wants to become the first woman elected to the White House.

“I’m running for president,” Ms Clinton said in a video posted on YouTube:

“Everyday Americans need a champion and I want to be that champion.

“Americans have fought their way back from tough economic times, but the deck is still stacked in favour of those at the top.”

Ms Clinton, who lost a bruising Democratic nominating battle to Barack Obama in 2008, is expected to travel soon to Iowa, the state that holds the kick-off nominating contest in early 2016.

Prior to Ms Clinton’s announcement, top aide John Podesta emailed party donors and supporters to say: “It’s official: Hillary’s running for president.”

“She is hitting the road to Iowa to start talking directly with voters. There will be a formal kick-off event next month,” he said.

Ms Clinton’s campaign will emphasise her plans to address economic inequality and will tout the historic nature of her effort to become the first woman US president, aides said.

One of her biggest challenges will be to show a more down-to-earth side while connecting with ordinary voters.

Critics, including liberals in her own party, have said the former US senator and secretary of state has grown out of touch after decades as the wife of former president Bill Clinton.

In a memo made public on Saturday, campaign manager Robby Mook told staff while the goal was for Ms Clinton to win the presidency, the campaign was not about her but about "everyday Americans".

"We are humble: we take nothing for granted, we are never afraid to lose, we always out-compete and fight for every vote we can win," he said in the campaign memo, titled We Are Hillary for America.

Republican candidates fire pre-emptive attacks

Even before the much-anticipated announcement, potential Republican opponents took swings at Ms Clinton.

As Hillary Clinton announces her bid to become the 45th — and first female — president of the United States, take a look at her political journey.

Former Florida governor Jeb Bush criticised her guidance of US foreign policy as secretary of state.

"We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies," Mr Bush said in a video released by the political action committee Right to Rise.

I'm running for president. Everyday Americans need a champion, and I want to be that champion. –H https://t.co/w8Hoe1pbtC

— Hillary Clinton (@HillaryClinton) April 12, 2015

Mr Bush, brother to former president George W. Bush, is currently exploring a presidential bid.

Kentucky senator Rand Paul made the rounds of Sunday talk shows to slam Ms Clinton's handling of a 2012 attack on a US diplomatic compound in Benghazi, Libya.

Mr Paul formally began his campaign for the Republican nomination last week.

He became the second Republican to announce his nomination after Ted Cruz did so in late March.

In her memoir Hard Choices, Ms Clinton dismissed the Republican criticism of her handling of the attacks as exploiting a tragedy for political gain.

Florida senator, Republican Cuban American Marco Rubio, is due to launch his campaign tomorrow.

Obama supports Clinton bid.

Ms Clinton's campaign-in-waiting has quietly organised for months, bringing on key staffers and advisers, plotting outreach operations and strategising.

She earned praise from Mr Obama on Saturday.

"She was a formidable candidate in 2008," Mr Obama said at a regional summit in Panama.

"She was an outstanding secretary of state. I think she would be an excellent president."

Madeleine Albright, who was secretary of state under Mr Clinton, tweeted that Ms Clinton is "going to be an outstanding president".

Ms Clinton leads opinion polls among Democrats, 60 per cent of whom said they would vote for her in the primaries, according to website RealClearPolitics.

Liberal political commentator and talk show host Tavis Smiley said Ms Clinton could use a Democratic challenger.

"I think the announcement was predictable, the nomination may be inevitable, but this campaign is not invincible," he said.

"And I think we learnt that the last time around, what will make her a better candidate is a challenge, and for some of us she is not our lover, but for some of us she is not progressive enough, for some of us she's too hawkish.

"And being pushed from her left flank makes her a better candidate, left to her own devices we see what the Clintons do and that is move to the centre and a lot of people are going to be disappointed in that."

Ms Clinton's first rally and the speech that kicks off her campaign will not take place until May, her team said.

ABC/wires