Mr. Trump may not have purposefully snubbed black neighborhoods — he rarely plunges into any community to tour businesses, sample local cuisine or spontaneously engage in the handshake and back-patting rituals of everyday campaigning. His preferred style of politicking consists almost entirely of addressing arena-size rallies, conducting media interviews and receiving visitors in private at events or at his Manhattan skyscraper.

But the 70-year-old white self-described billionaire has not just walled himself off from African-American voters where they live. He has also turned down repeated invitations to address gatherings of black leaders, ignored African-American conservatives in states he needs to win and made numerous inflammatory comments about minorities.

Mr. Trump’s mix of provocation and neglect has infuriated black Republicans, who fear that the party’s already dismal standing with African-Americans may sink so low that it barely registers in swing states.

“He’s alienated a number of minority voters, and that’s reflected in his low numbers,” said Tara Wall, a communications consultant who helped with black outreach on both of George W. Bush’s presidential campaigns and Mitt Romney’s 2012 bid. “You have to do the bare minimum, and he’s not even doing that.”