WASHINGTON (AFP) - Hillary Clinton is expected to announce her candidacy for US president Sunday, ending prolonged speculation that she once again seeks to become the first woman elected to the White House.

Seven years after her bitter nomination defeat to Mr Barack Obama, the former secretary of state and one-time first lady would enter the race as the Democratic Party's overwhelming favourite, as Clinton and her rivals gird for a bruising, 18-month campaign slog.

Her announcement is expected to be delivered via social media - perhaps in a tweet - and accompanied by a campaign kick-off video highlighting her economy-focused political philosophy, according to US news outlets citing sources close to Mrs Clinton operations.

It is likely to trigger a donor deluge from a vast network of 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.

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

On Saturday, the campaign team hunkered down in Clinton headquarters in Brooklyn, New York, where upcoming campaign manager Robby Mook reportedly delivered a memo to staff urging teamwork in the months ahead.

"We are a diverse and talented family... and have each other's backs," the memo states, according to Politico, perhaps in an effort to avoid the infighting among top aides that marred Mrs Clinton's 2008 run.

The 2016 campaign goal, the memo adds, is "to give every family, every small business, and every American a path to lasting prosperity by electing Hillary Clinton the next president of the United States".

On Saturday, she earned high praise from Mr Obama himself.

"She was a formidable candidate in 2008. She was a great supporter of mine in the general election. She was an outstanding secretary of state. She is my friend," Mr Obama said at a regional summit in Panama.

"I think she would be an excellent president."

Her Sunday announcement is expected to be followed by low-key campaign swings through Iowa and New Hampshire, states that hold the first votes early next year to determine the parties' nominees.

Mrs Clinton, 67, already has a team of organisers in Iowa, a fertile political proving ground ahead of national elections.

After the campaign launch, Mrs Clinton 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 the website RealClearPolitics.

A humble approach may help dispel doubts about Mrs Clinton raised in recent weeks, after it was revealed she used a private e-mail 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 e-mails that she described as personal, then wiped her server clean.

Mrs Clinton would be the biggest political headliner to enter the race, although not the first.

Conservative Republican Senator Ted Cruz, 44, made a splashy presidential campaign launch last month, and on April 7 fellow Senator Rand Paul, a libertarian-leaning first-term Republican, threw his hat in the ring.

Mrs Clinton's rollout may well steal the thunder of another Republican senator aiming for the White House, Marco Rubio, who is scheduled on Monday to make his own all-but-certain campaign declaration.

Ex-Florida governor Jeb Bush, son and brother of two presidents, is exploring a run and would be considered a Republican frontrunner should he enter the fray.