Redistricting has expanded the district into majority Latino areas where voters are less familiar with all three candidates. The primary is also being held later than expected, on the day after Memorial Day, which could affect turnout.
Johnson has not debated her opponents and has held relatively few campaign events. She dismissed both in a radio interview last month.
"If you want to know my honest opinion, I don't think anybody who's running against me is ready to come to this job," Johnson said.
She has a coveted endorsement from Obama, who rarely intervenes in Democratic primaries. Johnson's campaign says she remains focused on the district by holding weekend events and working hard in Washington during the week.
"The way she has always approached it is, 'I'll run on my record, so the best thing I can do is continue to extend my record,'" spokesman Eddie Reeves said.
Johnson is a former chairwoman of the Congressional Black Caucus, an influential group that once counted Obama as a member. She's one of the oldest members of the CBC, which has at least two other members in tough re-election fights — Conyers and Rangel, who has been a political mainstay in Harlem since 1971.
Rangel, who was convicted on House ethics charges in 2010, faces two black Democrats in his bid for a 22nd term — local district leader Joyce Johnson and former Democratic National Committee official Clyde Williams, as well as Latino state Sen. Adriano Espaillat. Latinos make up more than half the population in Rangel's newly redrawn district. The primary is June 26.
Another longtime New York congressman, Edolphus Towns, declined to seek a 16th term. Towns, 77, would have faced a tough primary fight against two black Democrats; new areas also were added to his Brooklyn district after redistricting.
"People are challenging at every level, and I think it's not the worst thing in the world," said former Virginia Gov. Doug Wilder, the nation's first elected black governor. "Seniority is good to the extent that it produces, and in the absence of that, the question then becomes, what have you done for me today?"
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