The one-time first lady enters the race seven years after her bitter nomination defeat to President Barack Obama and the announcement is expected to trigger a donor deluge from a vast network of supporters who have long waited for her to officially enter the 2016 presidential race.

"I'm running for president. Everyday Americans need a champion, and I want to be that champion," Clinton made the announcement on Twitter.

In an accompanying video, Clinton pledged to be a champion for everyday Americans and their families.

She said she believes this campaign is about voters, not her, and urged people to get involved in her campaign saying, "It's your time."

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The announcement came first in a tweet with an accompanying video and then in an email to her supporters. The video features middle class families talking about their lives and planning for what is ahead.

In the video, Clinton explained why she is running, saying, "Americans have fought their way back from tough economic times but the deck is still stacked in favour of those at the top."

She argued for an economy where "you can do more than just get by, you can get ahead and stay ahead, because when families are strong, America is strong."

The wife of former president Bill Clinton said she is committed to earning every vote and is starting with a focus on the early Democratic primary states. She will start with stops in Iowa this week to talk with Iowa voters, ramping up to a campaign kickoff in mid-May.

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The Democratic Congressional Campaign Committee Chairman Ben Ray Lujan welcomed the announcement.

"If chosen as our presidential nominee, Hillary Clinton will bring experience and energy to the campaign trail that will turn out hard-working families in New Mexico and across the country, and excite a new generation of Democrats," Lujan said.

The Democratic National Committee Chair Debbie Wasserman Schultz welcomed Clinton as the first official candidate for President of the United States to seek the Democratic Party's nomination in 2016.

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"As First Lady, Senator, and Secretary of State, Hillary Clinton has been a forceful advocate for women, children, and families, and an effective ambassador on the global stage," Schultz said.

A day earlier, Obama had told reporters in Panama that Clinton would be a great president.

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