For seven decades, two African-American politicians-first Adam Clayton Powell Jr., then Charles Rangel-have represented Harlem in Congress, symbolizing the New York City neighborhood's status as the de facto capital of black America.
Now, redistricting under way by the state legislature combined with a fast-rising Hispanic population are threatening to overturn that history. There are more Hispanics than blacks in Mr. Rangel's district, raising the prospect that Harlem's roughly 200,000 African-Americans will lose their dominant role in choosing the district's member of Congress.
Similar issues are emerging in several areas around the country, including Southern California, where the growing Hispanic population poses a challenge to three black-held seats.
"It's definitely becoming harder across the country to create as many majority African-American districts as were created in the 1990s and 2000s, and if you really can't create a majority African-American district based on Harlem anymore, that's pretty dramatic," said Richard Pildes, a New York University professor whose writing on district lines has been cited by the Supreme Court.
Sharpening the reversal of fortunes is the role of the Voting Rights Act of 1965. Originally designed to end antiblack discrimination at the polls, the law now could help Hispanics gain control of the Harlem seat. That's because the law protects minority groups' ability to elect representatives of their choice.
The 81-year-old Mr. Rangel has represented Harlem since 1971 and, thanks to the power of incumbency, is given a good shot of winning in 2012 even after he was censured by Congress for ethics violations. The redistricting is more likely to affect elections after Mr. Rangel retires.
"The changes in population cause a lot of problems to different groups," Mr. Rangel said in an interview. "To tell you the truth, I really don't know how it's going to be done."
According to 2010 census data on his district's voting-age citizens, 27% are non-Hispanic whites, 33% are black and 35% are Hispanic.
Mr. Rangel's district has sent a black person to Congress since World War II, when redistricting created a black-dominated district with Harlem at its heart. Since the days of Tammany Hall, politicians in New York have tapped racial and ethnic voting patterns to amass power.
Lawmakers and judges are struggling to adapt the Voting Rights Act to a nation that has seen major growth in Hispanic, Asian and ethnically mixed voters. Mr. Rangel's district reflects these challenges. The district's 2010 Democratic primary pitted a legendary black politician-Mr. Rangel-against a Hispanic politician with the name of a legendary black politician-Adam Clayton Powell IV, born to Mr. Powell Jr. And a Puerto Rican mother, who raised him on the island.
Privately, some current and former aides to black representatives say they fear the coming decade will, through redistricting and retirements, gradually decrease the number of African-Americans in Congress.
Redistricting specialists say keeping Harlem part of a district where blacks have the edge would require awkward line-drawing, such as reaching far north into the Bronx or even outside New York City to pick up other black-majority areas along with some white neighborhoods.
"You cannot create a majority black district in Harlem," said Columbia University's Nathaniel Persily, a veteran of past line-drawing efforts. "There's just not enough [blacks] there.''
Hazel Dukes, president of the New York chapter of the National Association for the Advancement of Colored People, said far-reaching district lines may be needed so as not to "weaken the black voice" in national politics. In testimony submitted to a redistricting panel, Ms. Dukes called the Harlem district a "powerhouse in our community and in Washington."
Esmeralda Simmons, executive director of the Center for Law and Justice at Medgar Evers College, said she understands that motive but doesn't agree the answer is a new district that may include New York suburbs. "That would destroy an existing district to create a new black district," she said.
Mr. Rangel, for his part, has long opposed his district's lines veering outside of Manhattan.
In Southern California, the rise of Hispanic and Asian populations forced a state redistricting panel to diminish black voting strength in three districts held by black lawmakers. One of them, Rep. Laura Richardson, now faces a primary challenge from a white opponent.
"None of our districts are majority black," said Marqueece Harris-Dawson, head of the African-American Redistricting Collaborative, which tried to keep three black districts in the area. "Our candidates have had to build multiracial coalitions, and that's going to become more of a reality for people across the country."
Columbia's Mr. Persily said similar tensions have cropped up in Chicago, Miami and some parts of the South.
In New York City, Hispanic lawmakers already hold two congressional seats and would like get a third-even as the state loses two of its 29 seats because of slow population growth.
"Anything that does not acknowledge the growth of this community... betrays the mission of what the census is all about," said Guillermo Linares, a Dominican-American state legislator. Mr. Linares is among those pressing for a Hispanic-focused district that would stretch from upper Manhattan to the Bronx.