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“I think they’ll both have strengths in the African-American community, but to get the numbers that you need to get in a crowded field, they’ll both need to have crossover appeal,” said Stephen Benjamin, the mayor of Columbia, S.C. Mr. Benjamin has engaged with both candidates, but is waiting until it gets closer to his state’s primary in February 2020 to make an endorsement.

“So, yes, it’s a collision course between the two African-American candidates, but they must also carve out their own lane,” he said.

Former Vice President Joseph R. Biden Jr. has tried to position himself as the continuation of President Barack Obama’s legacy since entering the race last week, and enjoys deep relationships with black leaders in the Democratic Party. In several national polls, which can provide an early reading of the electorate, Mr. Biden leads among all demographics, including black voters. Ms. Harris has slipped to single digits in recent weeks after a strong opening month, and Mr. Booker continues to languish.

For Ms. Harris and Mr. Booker, the path to the 2020 presidential race began more than a decade ago. At the Democratic National Convention in 2008, the two barrier-breaking black officials were featured during a luncheon for rising stars in a party that was already being reshaped in Mr. Obama’s image. Ms. Harris at the time was the San Francisco district attorney; Mr. Booker was in his first term as mayor of Newark.

Ms. Harris’s election to the Senate in 2016 raised her national profile and established her as a likely presidential contender. Mr. Booker won his Senate seat in 2013, cementing his national ascendence.