The question of just how many young African-Americans will show up to vote carries profound implications for this election. Mrs. Clinton is sure to dominate Mr. Trump among black voters, but her overwhelming margin could ultimately matter less than the total number of blacks who show up to vote.

To replicate President Obama’s success in crucial states such as Florida, Ohio and Pennsylvania, she cannot afford to let the percentage of the electorate that is black slip far below what it was in 2012. And while a modest drop-off of black votes may not imperil Mrs. Clinton’s prospects, given Mr. Trump’s unpopularity among upscale white voters, it could undermine Democrats’ effort to capture control of the Senate and win other down-ballot elections.

Mrs. Clinton’s difficulties with young African-Americans were laid bare in four focus groups conducted in Cleveland and Jacksonville, Fla., for a handful of progressive organizations spending millions on the election: the service employees union, a joint “super PAC” between organized labor and the billionaire environmentalist Tom Steyer, and a progressive group called Project New America. The results were outlined in a 25-page presentation by Cornell Belcher, a Democratic pollster, and shared with The New York Times by another party strategist who wanted to draw attention to Mrs. Clinton’s difficulties in hopes that the campaign would move more aggressively to address the matter.

Word of the report has spread in the constellation of liberal operatives and advocacy groups in recent weeks, concerning officials who saw diminished black turnout hurt Democratic candidates in the last two midterm elections.