In nearly simultaneous speeches in churches with black congregations less than a block apart, Obama and Clinton touted the heroes of "Bloody Sunday" for paving the way for their landmark drives to become, respectively, the first black and woman president.
 
"I stand on the shoulders of giants," Obama said in a packed ceremony in the church used as a headquarters before the march by the civil rights leader the Reverend Martin Luther King.
 
"I'm here because somebody marched for freedom. I'm here because you all sacrificed for me," he said.
 
'We shall overcome'
 
Clinton, speaking at a packed Baptist church down the street, said the voting rights won after the march and the bloody confrontation with state troopers had helped fuel her career along with Obama and New Mexico Governor, Bill Richardson - another presidential contender.
 
"It is giving Senator Obama a chance to run for president, Bill Richardson, a Hispanic, a chance to run and it is giving me a chance," she said to cheers.
 
"I know where my chance came from, and I am grateful."
 
Both services ended with the candidates linking arms with the other speakers to join the audience in singing the hymnal and civil rights anthem "We Shall Overcome."
 
The early campaign collision between Clinton and Obama, the top two contenders for the 2008 Democratic presidential nomination, is another sign of the budding intensity of their rivalry and the importance of their duel for black votes in the early primaries.