This was supposed to be the post-Barack Obama presidential election when Republicans regained a modest share of the black vote, but Donald Trump has watched support among these voters fade nationwide and in battleground states where minorities could play a decisive role.

The Republican presidential nominee is missing a golden opportunity to make inroads to black communities. Worse yet for the New York real estate mogul, he is driving black voters to Democratic nominee Hillary Clinton — in one survey he attracted just 1 percent of blacks — by appearing racially insensitive or even intolerant, warned Republican campaign veterans.

“The Muslim issues — the same thing with attacking the [Mexican-American] judge — it makes him look intolerant,” a Republican strategist said privately. “The worst thing you can call anybody in society these days, other than a murderer or a rapist or a child molester, is a racist.”

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Early on, Mr. Trump had an opening with black voters, who, like the majority of all voters, said they don’t trust Mrs. Clinton. But the opening rapidly closed as black voters increasingly came to distrust Mr. Trump.

Mr. Trump touts his support among minorities and has made overt appeals to black voters, including making the case that President Obama’s policies have left them behind with unemployment rates double that of whites.

“I will do more for the African-American people than Barack Obama has ever done,” Mr. Trump declared at a recent rally in Phoenix.

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He also featured black supporters at rallies, such as appearing during the primary race with devoted fan Jamiel Shaw Sr., whose 17-year-old son was shot dead outside their Los Angeles home in 2008 by an illegal immigrant who remained in the U.S. despite a history of gun crimes.

Last month, he hired Omarosa Manigault, a star from his former reality competition TV show “The Apprentice” on NBC, as the campaign’s director of black outreach.

She joined Mr. Trump’s daughter-in-law, Lara Trump, on Sunday at an “African-American Church Town Hall Rally” in Charlotte, North Carolina.

However, as often as Mr. Trump reaches out to black voters, he alienates them.

“Look at my African-American over here! Look at him!” Mr. Trump called out, pointing to a black man in the crowd at a rally in California.

The man, who said that he wasn’t a Trump supporter but came to the rally to check it out, said that he didn’t take offense. But critics in the news media were quick to interpret Mr. Trump’s shoutout as racist.

“It’s going to be hard for him to make a credible case that he is not racially [insensitive] or racially intolerant when you can point to what he says chapter and verse,” said Emory University political science professor Andra Gillespie, who specializes in racial politics at the Joint Center for Political and Economic Studies in Washington.

Ms. Gillespie said Mr. Trump sends all the wrong signals to black voters: throwing Black Lives Matter protesters out of his events, defending supporters who manhandle or sucker punch black demonstrators and calling himself a “law and order” candidate, which she said sounds like a racial dog whistle.

The professor said it is “highly questionable” that Mr. Trump could capture 10 percent of the black vote given the tone of his campaign.

Mr. Trump’s support among black voters hit rock bottom in a national poll last week showing Mrs. Clinton monopolizing that demographic 99 percent to 1 percent.

The overwhelming support from black voters, who are expected to constitute 12 percent of the electorate in November, helped Mrs. Clinton top Mr. Trump overall, 47 percent to 38 percent, in the NBC News/Wall Street Journal poll.

The same poll in May showed Mr. Trump with 9 percent of the black vote, compared to Mrs. Clinton with 88 percent. Back then, Mr. Trump was edging closer to the 10 percent mark that Republican candidates averaged before Mr. Obama, the country’s first black president, who garnered 95 percent in 2008 and 93 percent in 2012.

Mrs. Clinton overall had a thin 3 percentage-point lead in the May survey.

A recent battleground survey by the same pollsters showed Mr. Trump with zero support among black voters in Ohio and Pennsylvania, although the small number of minorities in the polling sample likely contributed to the complete absence of support.

Republican pollster Jim McLaughlin encouraged Mr. Trump to make forays into black communities and deliver his message directly to minority voters.

“It presents a great opportunity, mainly because of Hillary Clinton’s negatives. She’s not nearly as popular as Barack Obama is among Latinos and African-Americans,” he said. “They are looking for change just like everybody else is, whether it comes to the economy, education, crime, drugs.”

Mr. Trump has mostly avoided putting himself in front of minority audiences.

He declined an invitation to address the NAACP in Cincinnati last month while the Republican National Convention was underway in Cleveland.

“We were hoping he would make the short trip from Cleveland to Cincinnati,” NAACP President Cornell William Brooks told CNN at the time.

The last Republican to skip the NAACP conference in an election year was President George W. Bush in 2004, who was the first to miss it since Ronald Reagan in 1980.

Herman Cain, a leading black conservative supporting Mr. Trump, said his candidate’s message resonates strongly with black voters without him staging photo ops in black communities.

“The media is not going to give him credit for that. If they are looking for him to pander to the black community, he’s not going to do that. That’s not his style,” said Mr. Cain, a businessman who ran for the Republican presidential nomination in 2012 and currently hosts a nationally syndicated talk radio show.

“Donald Trump is not about pandering. Hillary Clinton is about pandering,” he said. “The black people that are conservative. They know what he stands for.”