Hillary Clinton cast herself as a champion for everyday Americans on Sunday, kicking off her long-awaited second run for the White House with a vow to fight for a level playing field for those recovering from tough economic times.

Clinton, who begins the 2016 presidential race as a commanding Democratic front runner, entered the fray with a video announcement in which she said the economic deck was still stacked for those at the top.

"Everyday Americans need a champion, and I want to be that champion," she said in the video, which was posted on her new website on Sunday afternoon. "So you can do more than just get by, you can get ahead. And stay ahead."

Clinton, who lost a bruising Democratic nominating battle to Barack Obama in 2008, said in a tweet that she would be travelling to Iowa, the state that holds the kickoff contest in the parties' nominating process in early 2016.

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 the biggest challenges for a woman who has been one of the most famous figures in the United States since the early 1990s, will be to show a more down-to-earth side while connecting with ordinary voters.

Critics, including liberals in her own party, say she has grown out of touch after decades as the wife of former President Bill Clinton, a US senator and secretary of state.

To address that, Clinton's website and the announcement video feature Americans talking about their futures, and an image of her holding a paper coffee cup at a table with a couple of elderly people.

Clinton is then seen listening to voters before the video cuts to her speaking outside a home. "I'm getting ready to do something too, I'm running for president," she says, before emphasizing to voters that it is "your time."

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

Within a half hour of Clinton's announcement, the campaign kickoff video posted to her Facebook page had been viewed nearly 90,000 times and she topped the list of topics trending on Twitter.

The campaign is aware of the pitfalls of star power. In a memo made public on Saturday, Clinton's campaign manager, Robby Mook, told staff to stay humble.

"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.

PRE-EMPTIVE ATTACK

Even before Sunday's much-anticipated announcement, potential opponents in what is shaping up to be a crowded Republican presidential field took swings at Clinton.

Former Florida Governor Jeb Bush criticized 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," Bush said in a video released by the political action committee Right to Rise.

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

Kentucky Senator Rand Paul, who formally began his campaign for the Republican nomination last week, made the rounds of Sunday talk shows to slam Clinton's handling of a 2012 attack on a U.S. diplomatic compound in Benghazi, Libya.

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

Many Democrats have been waiting for Clinton to get back into the White House fight since the day in June 2008 when she pulled out of her primary battle against Obama with an expression of regret that she could not crack "that highest and hardest glass ceiling this time."

But Clinton still has to convince some liberals that she is the best candidate to tackle issues like income inequality and the power of Wall Street banks. Some liberal groups are pushing Democratic Senator Elizabeth Warren, who has vocally criticized some Wall Street practices, to challenge Clinton.

Richard Trumka, head of the AFL-CIO, the largest US federation of labour unions, praised Clinton's public service record and said he hoped her candidacy would elevate the "critical debate" in the country over how to raise wages.

The Clinton campaign's finance chair, Dennis Cheng, emailed donors and bundlers on Sunday telling them to expect an email message from Clinton herself, one donor said. Cheng's email, according to the donor, said Clinton would be explaining her vision for the campaign and her presidency.

Marc Stanley, a Dallas lawyer and a prominent Democratic fundraiser, said he and a colleague planned to send "several hundred" messages to donors on Sunday asking them to support Clinton.