Hillary Clinton sets off for tour of key states

Mrs Clinton's team said she would spend the next few weeks building up grassroots support in the early Democratic primary states.

Her first rally, to officially kick off her campaign, is not expected until mid-May. But her trip to Iowa is to be a "listening tour" where Mrs Clinton will meet voters at low-key events.

Later this week, she is expected to meet groups of students, teachers and small business owners.

On Sunday, Mrs Clinton launched her campaign website and declared in a video that she was running for president.

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

She added that she wanted to be a champion for "everyday Americans".

The video features a number of Americans talking about their hopes and aspirations.

Analysis: Gary O'Donoghue, Des Moines, Iowa

The launch of Hillary Clinton's campaign for the presidency was meant to be low-key. It was meant to reflect the idea that it was all about ordinary Americans and the everyday concerns of the middle class.

But whether she likes it or not, Mrs Clinton is a huge name in politics and reinventing herself as the embodiment of change won't be easy.

True, she is unlikely to face any stiff competition from her own side for the nomination, but Republicans have been unrelenting in their attacks on her and that will only increase.

She will also have to find a convincing vision for America that will capture the imagination of the voters. Elections are usually about the future and Mrs Clinton has to prove that she's not all about the past.

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Is this Hillary Clinton's time?

Mrs Clinton, 67, has already had the backing of Mr Obama, who said on Saturday that she would make an "excellent president".

But the attacks from Republicans have already started.

Jeb Bush, former Florida Governor and brother of George W Bush who expected to stand as a Republican candidate, tweeted: "We must do better than Hillary."

In his own online video on Sunday, he focused on foreign policy, saying: "We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies."

Another Republican presidential contender. Rand Paul, also criticised Mrs Clinton for her handling of a September 2012 attack on a US diplomatic compound in Benghazi, Libya, in which the US ambassador was among those killed.

He also said questions remained about funds received by a charity set up by Mr and Mrs Clinton.

Republican Senator Marco Rubio is expected to announce his candidacy for his party's nomination at an event in Miami later on Monday.