After years of speculation, Hillary Clinton has announced she will run to be US president for a second time, saying: "Everyday Americans need a champion. I want to be that champion."
Clinton has been expected to make a second bid for the presidency since Barack Obama appointed her Secretary of State after defeating her in 2008.
If elected, she would be the first woman to be US President, the most powerful elected office in the world.
As she did in 2007, Clinton began her campaign for the nomination with a video.
"So I'm hitting the road to earn your vote. Because it's your time. And I hope you'll join me on this journey," Clinton said at the end of a video, which features people describing their aspirations.
She plans to head to the early-voting states of Iowa and New Hampshire, looking to connect with voters directly at coffee shops, day care centres and some private homes.
Only after about a month of these events will Clinton will give a broader speech outlining more specifics about her rationale for running.
The former secretary of state, senator and first lady enters the race in a strong position to succeed her rival from the 2008 campaign. Obama said she would be "an excellent president".
Clinton will face pressure from left of her party to adopt a more populist economic message focused on inequality. Some liberals remain sceptical of Clinton's close ties to Wall Street donors and the centrist economic policies of her husband's administration. They have urged her to back tougher financial regulations and tax increases on the wealthy.
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"It would do her well electorally to be firmly on the side of average working people who are working harder than ever and still not getting ahead," said economist Robert Reich, a former labour secretary during the Clinton administration who has known Hillary Clinton for nearly five decades.
The Republicans did not wait for her announcement to begin their campaign against her. The party's chairman, Reince Priebus, has outlined plans for a broad effort to try to undermine her record as secretary of state while arguing that her election would be like giving Obama a "third term."
Republicans have jumped on Clinton's use of a personal email account and server while she was secretary of state, as well as her handling of the 2012 terrorist attack in Benghazi, Libya.
Former Florida governor Jeb Bush, brother of former president George W. Bush, said in his own online video, on Sunday: "We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies."
Clinton would be the first woman elected president
Clinton is the first Democrat to get into the race, but there are some lower-profile Democrats considering challenging her.
The party's nominee will have to overcome history to win election. In the last half-century, the same party has held the White House for three consecutive terms only once, during the administrations of Republicans Ronald Reagan and George H.W. Bush.