Mrs. Clinton ran for the Democratic presidential nomination in 2008 but lost to Barack Obama. The overwhelming Democratic favourite, she had been expected to declare her candidacy for months. In a video on her website, Mrs Clinton declared: "I
am 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.
"Everyday Americans need a champion and I want to
be that champion," she added. Mrs Clinton was also first lady when her husband Bill Clinton was president.
This time around, Mrs Clinton's path to the Democratic nomination appears much easier. Unlike 2008 there's no inspirational, once-in-a-generation opponent like Mr Obama waiting in the wings. There's not even a charismatic, battle-tested candidate like former vice presidential nominee John Edwards or a Hispanic governor with foreign policy chops like Bill Richardson in the field.
But if Mrs Clinton's nomination campaign will be easier, actually winning the presidency could be just
as difficult – or more so. Unlike 2008, the Democratic nominee will be defending eight years of her party's rule, with all the baggage that comes with it.
Instead of facing a Republican Party on its heels,
fresh from massive losses in both chambers of Congress, a nominated Mrs Clinton will have to defeat a Republican candidate with the political wind at his back.
She is now expected to travel to Iowa and New Hampshire, two early primary contests in the 2016 race.
After her failed nomination bid in 2008, Mrs Clinton
served as secretary of state in Mr Obama's first
administration (2009-2013). Known for her punishing travel schedule – she visited 112 countries in four years – she led the US response to the Arab Spring and the military intervention in Libya in 2011.
Mr Obama praised her, saying at a news conference
at the Americas summit in Panama on Saturday that
she would make an "excellent president". And her successor in the post, John Kerry, called her a "good friend", telling ABC's This Week programme she "did a terrific job of rebuilding alliances that had been shredded over the course of the prior years".
'Above the law' But Republican presidential contender Rand Paul 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.
"There is a history of the Clintons feeling they are above the law," the Kentucky senator said on CNN's State of the Union programme.
Two prominent Republicans have officially entered the race for their party's nomination – Mr Paul and Texas senator Ted Cruz. Former Florida governor Jeb Bush is another frontrunner.
As a senator, Mrs Clinton voted for the invasion of
Iraq in 2003 but distanced herself from the way the war was waged, and called for US troops to be withdrawn.
During her husband's first term as president, she campaigned for healthcare reform but her plan fell apart and never made it to a vote in Congress.
She was also embroiled in the some of the scandals
which marred her husband's presidency, becoming the only US first lady to be called to testify before a grand jury.
Mrs Clinton stood by her husband when he was exposed as having had an affair with a White House intern, Monica Lewinsky.
BBC.