Mr Lewis originally threw his support behind Mrs Clinton in October

The defection comes the day after the two main Democratic presidential candidates clashed in a TV debate.

Neither candidate landed a killer blow, but correspondents say Mrs Clinton failed to stop Mr Obama's momentum.

Mrs Clinton has been struggling to revive her campaign before critical primaries in Ohio and Texas.

'Tremendous pressure'

Mr Lewis, a congressman from Georgia and a veteran of the US civil rights movement, defended his switch of support by saying he wanted "to be on the side of the people".

"The people are pressing for a new day in American politics and I think they see Senator Barack Obama as a symbol of that change," he said in a statement.

He originally came out in support of Mrs Clinton's campaign for the Democratic presidential nomination in October.

Mrs Clinton said she thought Mr Lewis had been under "tremendous pressure" to jump ship to the Obama campaign.

Black voters in Georgia voted overwhelmingly for Barack Obama in the state's primary in February.

"At the end of the day it's not about who is supporting us, it's about what we're presenting, what our positions are, what our experiences and qualifications are," Mrs Clinton said.

Mr Obama said he was "deeply honoured" to have the support of Mr Lewis.

Clinton wins needed

His loss is a fresh blow to Mrs Clinton's campaign after Mr Obama's 11 straight primary and caucus victories.

Analysts say she needs to win a majority of delegates in the next big primaries in Texas and Ohio on 4 March to stay in the race.

The Democratic Party chooses its presidential candidate at the national nominating convention in August, ahead of the November elections.

In a televised debate in Cleveland, Ohio, on Tuesday, the two candidates accused each other of negative campaigning and attacked each other's policies on health care, trade and foreign policy, including the Iraq war.

It was hard to define an outright winner of the debate, says the BBC's Jonathan Beale in Washington.

But there was nothing to suggest Mrs Clinton did enough to turn the tide that has been moving in Mr Obama's favour, says our correspondent.