Hillary Clinton to hold farewell rally and throw her support behind Obama

Hillary Clinton is set to formally abandon her White House campaign at the end of the week after having bid an emotional farewell to her loyal staff

Clinton will also use the speech to urge once-warring Democrats to focus on the general election and defeating Republican presidential candidate John McCain.

Mrs Clinton surprised many on Tuesday when she refused to concede defeat after Barack Obama gained enough delegates to win the nomination.

A number of her supporters, including the New York member of Congress, Charlie Rangel, are said to be furious with Mrs Clinton for failing to acknowledge Obama's victory.

Mrs Clinton has told colleagues in her home state of New York that she would be willing to become Barack Obama's running mate if it would help Democrats win the White House.

Wiinner: Barack Obama claims victory in the race to be Democratic candidate for the White House

Mr Obama , however, has remained steadfastly non-committal about his rival receiving the vice-presidential nomination.

He told ABC News that Clinton is a 'special case' to consider as a candidate for the vice presidency.

'She's somebody who travelled this journey with me. She was extraordinarily capable and tenacious. I mean, she is just a great candidate,' Obama said.

He added: 'We're going to be having a conversation in coming weeks, and I'm very confident how unified the Democratic Party's going to be to win in November.'

On Tuesday a flurry of superdelegates – Democrat Party insiders who remained on the fence until the eleventh hour – sided with Mr Obama to push him past the magic figure of 2,118 delegates he needed to claim a famous victory.

He  will be crowned the Democratic nominee at the convention in August and will face Republican John McCain in November's election to choose a successor to President Bush.

The win by Mr Obama, son of a black Kenyan father and white mother from Kansas, marked a milestone in U.S. history. It came 45 years after the height of the civil rights movement led by Martin Luther King, who was assassinated in 1968.

It followed one of the closest and longest nomination fights ever. Mrs Clinton, who would have been the first woman nominee in U.S. political history, won more than 1,900 delegates.

Celebration: Barack Obama is mobbed by supporters in St Paul after declaring victory