SELMA, Ala. — The presidential campaigns of Senators Obama and Clinton crossed paths for the first time yesterday as they paid homage to civil rights activists who they said helped give them the chance to break barriers to the White House.

The two candidates and President Clinton, making his first appearance with his wife since her campaign began, linked arms with activists who 42 years ago were attacked by police with billyclubs during a peaceful voting rights march. “Bloody Sunday” shocked the nation and helped bring attention to the racist voting practices that kept blacks from the polls.

“I’m here because somebody marched for our freedom,” Mr. Obama, who would become the first black president, said from the Brown Chapel AME Church where the march began on March 7, 1965. “I’m here because you all sacrificed for me. I stand on the shoulders of giants.”

Not to be outdone in the hunt for black votes, Mrs. Clinton also spoke in Selma at a church three blocks away and brought a secret weapon — her husband. Three days before the march anniversary, her campaign announced that the former president who is so popular among blacks would accompany her for his induction into Selma’s Voting Rights Hall of Fame.

Mrs. Clinton said the Voting Rights Act and the Selma march made possible her presidential campaign, as well as those of Mr. Obama and New Mexico’s Governor Richardson, who would be the first Hispanic to occupy the Oval Office.

“After all the hard work getting rid of literacy tests and poll taxes, we’ve got to stay awake because we’ve got a march to continue,” Mrs. Clinton said in a speech interrupted numerous times by applause and shouts of approval. “How can we rest while poverty and inequality continue to rise?”

Mrs. Clinton and Mr. Obama both appeared outside Brown Chapel for a rally but came from opposite sides of the podium and did not interact. Despite the intense rivalry between their campaigns, the two praised each other.

“It’s excellent that we have a candidate like Barack Obama who embodies what all of you fought for here 42 years ago,” Mrs. Clinton said. Mr. Obama said Mrs. Clinton is “doing an excellent job for this country, and we’re going to be marching arm-in-arm.”

But they did not join arms when the commemorative march attended by thousands got under way. Instead, Mrs. Clinton held hands with her husband, and Mr. Obama was several people down the line. Mr. Obama, who shed his coat and tie for the march, approached Mrs. Clinton at one point, and the two chatted for a few seconds before moving back to opposite sides of the street.

The two candidates sounded similar themes in their speeches. Both said the civil rights movement is not over because inequality still exists in education, health care, and the economy. Both criticized the Bush administration for failing to return Hurricane Katrina victims to their homes.

But Obama, who was three years old on Bloody Sunday, delivered a call to action that would be politically unfeasible for Mrs. Clinton or any of his other white rivals. He said the current generation of blacks does not always honor the civil rights movement and needs to take responsibility for improving their lives by rejecting violence; cleaning up “40-ounce bottles” and other trash that litters urban neighborhoods, and voting instead of complaining that the government is not helping them.