WASHINGTON: Hillary Clinton finally announced her White House candidacy Sunday, a move that empowers her to parry Republican attacks as she seeks to become the United States' first female president.

Seven years after her bitter nomination defeat to Barack Obama, the former secretary of state and first lady jumps into the race as the Democratic Party's overwhelming favorite, as Clinton and her rivals gird for a bruising 18-month campaign.

“I'm running for president,” a beaming Clinton said in a video on her campaign website that went live at about 3:00 pm (1900 GMT) Sunday.

“Everyday Americans need a champion, and I want to be that champion, so you can do more than just get by. You can get ahead, and stay ahead,” she said, after a two-minute clip featuring middle-class and working-class couples and families sharing their aspirations.

Her nascent campaign emailed supporters saying Clinton will spend “the next six to eight weeks in a 'ramp up' period,” building a grass-roots organisation and “engaging directly with voters. “Her first rally and the speech that kicks off her campaign will not take place until May, her team said.

Clinton will first head to Iowa, the state that holds the debut vote early next year to determine the parties' nominees. “I'm hitting the trail to earn your vote,” she said.

The announcement, which meets no substantial challenge from other Democrats, will no doubt trigger a donor deluge from supporters who have long waited for her to officially enter the race, a move that would allow them to contribute directly to her 2016 election effort.

But it also brought an immediate wave of Republican opposition, including from the Republican National Committee, which said Clinton “has left a trail of secrecy, scandal, and failed policies that can't be erased from voters' minds.“

“Our next president must represent a higher standard, and that is not Hillary Clinton,” RNC chairman Reince Priebus said in a statement.

“We must do better than Hillary,” tweeted former Florida governor Jeb Bush, just one minute after Clinton posted her own announcement to her 3.2 million Twitter followers, in a likely foreshadowing of the intense back-and-forth expected to play out on social media in the run up to the November 2016 election.

'Barnstorm through Iowa'

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

On Saturday, she earned praise from Obama, although experts warn she will have to tread a fine line in how closely she aligns herself with the incumbent.

“She was a formidable candidate in 2008.... She was an outstanding secretary of state,” Obama said at a regional summit in Panama. “I think she would be an excellent president.“

The soft approach, a folksy video, small, low-key gatherings with heartland voters, would mark a deviation from the Clinton Inc. juggernaut that ultimately failed in 2008.

After the campaign launch, Clinton, 67, should “jump on a bus and barnstorm through Iowa touching all 99 counties and meet with people in cafes and other small venues” as she reintroduces herself to Americans, Iowa State University professor Steffen Schmidt told AFP.

The one-time US senator and wife of former president Bill Clinton leads opinion polls among Democrats, some 60 per cent of whom say they would vote for her in the primaries, according to website RealClearPolitics.

A humble approach may help dispel doubts about Clinton raised in recent weeks, after it was revealed she used a private email account while secretary of state from 2009 to 2013.

But she could face uncomfortable questions about the issue from voters, including why she deleted thousands of emails that she described as personal, then wiped her server clean.

'Unfit' to serve?

Clinton, who has been in America's political spotlight for a quarter-century, has endured heavy criticism from Republican rivals, and launching her campaign gives her a platform to aggressively counter their punches.

“There is sort of a history of the Clintons... feeling like they're above the law,” Senator Rand Paul, who announced last week he is running for president, told CNN Sunday.

On his website, Paul called Clinton “unfit to serve as president. “Conservative Senator Ted Cruz made his own splashy presidential campaign launch last month, while fellow Senate Republican Marco Rubio is scheduled to make his own all-but-certain campaign declaration on Monday.

On Sunday, Jeb Bush teased his own expected campaign rollout.

Just hours before Clinton's announcement, the son and brother of two former presidents released a video saying he would soon lay out his policy proposals.

“We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies,” he said.

Clinton leads against her GOP rivals in nearly all polls, but famed political prognosticator Nate Silver on Sunday called the 2016 election a “toss-up. “