A day ahead of Hillary Clinton's expected announcement of her bid Democratic party's 2016 presidential nomination, President Barack Obama hailed the former first lady, saying that she would make "an excellent president".

"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," Bloomberg quoted Obama as saying.

Clinton is likely to announce that she will enter the race for theDemocratic nomination on Sunday, April 12, via a video message on social media.

The 67-year-old's campaign is likely to centre on boosting economic security for the middle class and expanding opportunities for working families, two senior advisers have said.

According to the Press Trust of India, the two advisers also provided the first preview of the message that the former secretary of state has planned to convey when she launches her campaign.

Clinton lost the 2008 nomination to President Obama and, this time around, it appears she is unlikely to face a formidable primary opponent, with only a few low-profile Democrats saying that they would consider contesting the nomination.

Two other potential candidates, including senator Elizabeth Warren and vice-president Joe Biden, have not yet confirmed their candidature.

From the Republican side, Clinton could face senators Rand Paul and Ted Cruz, who have already thrown their hats in the ring. Former governor of Florida Jeb Bush -- brother of former president George W. Bush and son of George H.W. Bush -- is also likely to stand in the election.