After months of speculation, former first lady and senator Hillary Clinton has officially announced she wants to become America’s first female president.

Mrs Clinton, 67, made the announcement on Twitter on Monday morning, saying she wanted to be a champion for everyday Americans.

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The trained lawyer has already had a long career in politics, serving as first lady alongside husband Bill Clinton and as secretary of state under President Barack Obama.

In a slick video posted on her campaign website, Mrs Clinton said she was prepared for the challenge.

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

“Everyday Americans need a champion and I want to be that champion, so you can do more than just get by, so you can get ahead and stay ahead.”

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This is not the first time Mrs Clinton has sought election, unsuccessfully running against Mr Obama to be the Democratic Party presidential candidate in 2008.

Mr Obama will not seek re-election in 2016 after serving the presidential limit of two terms, and will step down before November next year.

Mrs Clinton’s daughter Chelsea congratulated her mother after the announcement, telling her 800,000 Twitter followers she was proud.

Speaking to Town and Country Magazine on Tuesday, former president Bill Clinton said his role in the lead up to the election would be as “backstage advisor” to his powerful wife.

If the former first lady is successful, Mr Clinton will be the country’s first male partner in the White House.

The campaign is expected to cost more than $1 billion.

I’m running for president. Everyday Americans need a champion, and I want to be that champion. –H https://t.co/w8Hoe1pbtC

— Hillary Clinton (@HillaryClinton) April 12, 2015

Very proud of you Mom! @HillaryClinton: http://t.co/PvMlg82cU2

— Chelsea Clinton (@ChelseaClinton) April 12, 2015

One day before the announcement, President Obama said Mrs Clinton would make “an excellent president”.

Speaking at a press conference in Panama, President Obama said “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. I think she would be an excellent president”.