“When a lot of white Republicans get together and bring up race, even telling black people how they should see police and the world, it evokes the worst kind of emotion,” said Mr. Hill, who supports Mr. Trump but decided to skip the convention. “We have so few black Republicans to begin with. Talking about race won’t bring us more.”

For many black Republicans, the party’s convention has veered unexpectedly and unhappily toward lecturing and moralizing on issues of race, an off-putting posture at a time when Mr. Trump is staggeringly unpopular with minority voters. He drew support from zero percent of African-Americans in recent NBC News/Wall Street Journal polls in Ohio and Pennsylvania, and he is struggling badly with Hispanics, partly because of his harsh language about Mexicans and immigrants.

These Republicans said they had preferred the political messages to black voters at recent conventions, where the focus was less on public safety and crime than on economic opportunity, job creation, support for small businesses and school choice — all issues, they said, that held appeal.