Former President Barack Obama traveled to Pittsburgh on Thursday to urge voters there to choose Vice President Kamala Harris in November, aiming a message at one group in particular: Black men.

The decision voters have between the vice president and former President Donald J. Trump, her Republican opponent, “isn’t a close call,” Mr. Obama said as he visited with a group of campaign volunteers and officials at a field office just ahead of his appearance at a Harris rally. His message was for Black male voters whom he said might not be yet on board with Ms. Harris.

Citing “reports I’m getting from campaigns and communities,” he called out what he said was flagging enthusiasm for Ms. Harris compared with the support he received when he was running for the presidency in 2008.

“You’re coming up with all kinds of reasons and excuses,” Mr. Obama said. “I’ve got a problem with that.

“Part of it makes me think that, well, you just aren’t feeling the idea of having a woman as president, and you’re coming up with other alternatives and other reasons for that,” Mr. Obama continued, adding that the “women in our lives have been getting our backs this entire time.

“When we get in trouble and the system isn’t working for us, they’re the ones out there marching and protesting.”

The stern words from the former president were meant to address worrying signs for Ms. Harris, including that her support among Black voters is still lower than what President Biden received when he won the state in 2020, according to a poll last month from The New York Times, The Philadelphia Inquirer and Siena College. Ms. Harris’s advisers and a raft of Democratic strategists believe that if anyone can lift Black voter turnout, it is Mr. Obama.

“He’s got, obviously, tremendous appeal to Black voters,” the Democratic strategist James Carville said. “He has tremendous appeal to suburban whites, which is another big part of the coalition. And he drives Trump nuts.”

Early voting has already begun in Pennsylvania, which Ms. Harris must almost certainly win to defeat Mr. Trump. She holds a narrow lead in the polls there, having overcome the significant deficit she inherited from Mr. Biden. Democrats are hoping for high voter turnout in the state’s biggest cities, Pittsburgh and Philadelphia.

Mr. Obama remains adored by Democrats, making him a natural surrogate for Ms. Harris on the campaign trail. More than 90 percent of Democrats and many independents view him favorably, according to an August survey by The Economist and YouGov — well above other Democrats, including Mr. Biden, Bill Clinton and Hillary Clinton.

Ms. Harris was an early supporter of Mr. Obama, flying to Iowa in 2007 to knock on doors for the junior senator from Illinois when she was the district attorney of San Francisco.

Erica L. Green reported from Pittsburgh and Katie Rogers from Washington. Nicholas Nehamas contributed reporting.