From here, she flew to a Mexican-American enclave, the East Los Angeles neighborhood, to eat at King Taco, ordered in Spanish by Mayor Antonio R. Villaraigosa, who has emerged as an important supporter.
In Chicago, Mr. Obama was successful in rallying Hispanics to his side and bridging differences with black voters. His success in repeating that is critical.
In 2004, Hispanics accounted for 16 percent of the vote in the California primary; 11 percent in New York, 17 percent in Arizona and 9 percent in Florida. Should he win the presidential nomination, his success at overcoming the history between the two groups will be critical as the Democrats approach an election in which they are looking to lock up the Hispanic vote for decades to come.
As he campaigned in northern Nevada, Mr. Obama acknowledged the challenges he faced.
“I think it’s important for us to get my record known before the Latino community,” he told reporters. “My history is excellent with Latino supporters back in Illinois, because they knew my record.
“Nationally, people don’t know that record quite as well. So it’s very important for me to communicate that, to advertise on Spanish-speaking television, to make clear my commitments.”
Relations between blacks and Latinos vary from place to place and have evolved over the years. Mr. Villaraigosa lost his first effort to become mayor, in 2001, to a white, James Hahn, who won 80 percent of the black vote. In a rematch in 2005, Mr. Villaraigosa was elected with 50 percent of the black vote.
Mr. Villaraigosa said he did not think that Mrs. Clinton’s strength among Hispanics was a product of tensions between the two groups.
“From my vantage point,” he said Monday, “the strength that Hillary Clinton enjoys among Latinos has everything to do with her track record and her longstanding relationship with that community. I think there are tensions among all groups.”
Gov. Bill Richardsonof New Mexico, who just dropped his own campaign to become the first Latino president, said that the rivalry between the two groups had eased and that Mr. Obama could transcend many of the differences as he approached Feb. 5, “what I call the Hispanic primary day.”
He said he might endorse a candidate before Feb. 5, an endorsement that would clearly influence some Hispanics.
If attitudes are shifting, they are moving slowly as so many racial assumptions are challenged.
The Rev. Al Sharptonof New York, who has been on the front line of many of the black-Latino battles in New York politics, said the tension would be a problem for Mr. Obama across the country and in New York, which also votes on Feb. 5. He said Mr. Obama would be at a disadvantage because of his choice to be a “race-neutral candidate.”
“It’s going to be a challenge that he has got to deal with,” Mr. Sharpton said. “There’s a natural history, and we’ve made some progress. But he has not been part of those efforts to make progress.”
In California, Mr. Obama has won backing from Latino lawmakers, some of whom had supported Mr. Richardson, but winning rank-and-file voters will be hard, said the State Senate majority leader, Gloria Romero, Democrat of East Los Angeles.
“Do we have a long way to go?” she asked. “Absolutely. I think there are some tensions on questions of immigration and jobs. But I believe that we have moved forward in a way that the community will embrace an African-American president.”
She said the solution to overcoming the tensions was discussing economic problems of middle- and lower-class blacks and Hispanics like the mortgage crisis, an issue that first Mrs. Clinton and now Mr. Obama have been raising more frequency.
“I don’t think eating tacos,” is effective, she said with a flick at Mrs. Clinton. “We need to address what unites us. The key is not to raise the wedge issue.”
Mr. Obama, some party officials and scholars suggested, may face additional difficulty if Hispanic women respond to Mrs. Clinton’s increasingly strong appeal for support based on sex. A rally here Saturday was packed with Hispanic women who shrieked at seeing Mrs. Clinton.
“The Hispanic community is very family oriented, and we respect our mothers,” said Ruben Kihuen, an influential Democratic assemblyman from Las Vegas who supported Mrs. Clinton. “A lot of middle-aged women see her as a mother, a head of the household, and they can identify with this. Especially when they see her daughter, Chelsea, with her.”
The tensions between Hispanics and African-Americans have increased proportionately with the influx of new Hispanics in areas like the Southwest, experts on the relationships said.
Mexican-Americans and other groups have increasingly migrated to traditionally black neighborhoods, the experts said.
“There have been enormous misunderstandings and conflicts over local resources and political representations between the two groups which simmer right below the surface and sometimes erupt,” said Albert M. Camarillo, founding director of the Center for the Comparative Study of Race and Ethnicity at Stanford.
Hispanic voters, Mr. Camarillo said, “might not go into the direction of the Obama camp.”
