President Trump has labeled former Vice President Joseph R. Biden “Sleepy Joe,” and some Democrats in early voting states are sorry to say they agree.

The 76-year-old third-time candidate’s uneven performance on the stump has unearthed lingering concerns about whether Mr. Biden has the stamina to beat Mr. Trump, much less to serve as president should he win the election.

“I hate to say that, but it was very uninspiring,” Ginnie Swarm told The Washington Times after catching a Biden speech in Iowa this month. “I don’t need fire and brimstone, but we do need to be fired up. I want to be excited about someone.”

Mr. Biden will get a prime-time opportunity to assure Ms. Swarm and other skeptical voters next week in the first debate of the Democratic primary season.

Since he entered the race in late April, Mr. Biden has been dogged by the concerns and has fueled them by keeping a lighter schedule than many of his rivals.

He has skipped some of the multicandidate gatherings and instead put efforts into big-dollar fundraisers that grassroots activists loathe. Those activists have also been irked that Mr. Biden won’t embrace the left-wingers’ top policy priorities.

Rep. Alexandria Ocasio-Cortez, a New York Democrat and bellwether for liberal activists, said early on that Mr. Biden doesn’t “animate her” and more recently stressed that Democrats should have learned from 2016 that they need a candidate who can electrify voters.

“I think that we need to pick a candidate that is going to be exciting to vote for, that all people, women, people of all genders, races, income levels, geographies feel excited and good about voting for,” she said on ABC. “That’s really what we should be looking for.”

President Trump also senses weakness.

“Joe Biden — he looks like he is just exhausted,” Mr. Trump said on the Spanish-language network Telemundo. “I don’t know what happened to him, but he is exhausted and he doesn’t do any work. He’s not working.”

The line of attack is reminiscent of the 2016 Republican primary when Mr. Trump branded his rivals in negative terms. He tagged former Florida Gov. Jeb Bush as “low-energy.”

Republican Party strategist Ana Navarro, a vocal Trump critic, suggested that the president’s attack against Mr. Biden could stick.

“I say this as someone who really likes Joe Biden,” Ms. Navarro said recently on CNN. “Right now, he is giving me Jeb Bush acid reflux. I want to see him pull this together.”

Mr. Biden’s lackluster stage presence was on display when he used a video message to address a candidate forum in Charleston, South Carolina, hosted by the Black Economic Alliance.

“No one knows more than the Black Economic Alliance how marred the history of the United States is by discrimination and exploitation of people of color,” Mr. Biden said in the video as people in the audience talked and milled about the Charleston Music Hall, barely acknowledging the former vice president’s speech.

When he concluded, no one applauded.

The audience paid more attention to the video messages from two other candidates who did not attend: Sen. Kamala D. Harris of California and Sen. Bernard Sanders of Vermont, who earned smatterings of applause.

Retired teacher Kay Haun, 81, said she had a hard time trusting Mr. Biden and preferred candidates who had a spark, such as Pete Buttigieg, mayor of South Bend, Indiana, former Rep. Beto O’Rourke of Texas and Sen. Elizabeth Warren of Massachusetts.

She described Ms. Warren as “personality plus.”

Voters who turned out for a recent Biden event in Iowa shared a similar sentiment about Ms. Warren.

Lory Chaplin, a retired high school art teacher, said she sees a “lot of energy coming from Elizabeth, a lot of excitement, just even in her physical movements.”

“She really draws you into her excitement,” said Ms. Chaplin, 64. “I love to hear her speak. She’s inspiring.”

Ms. Chaplin said she is split between Mr. Biden and Ms. Warren but openly worried that “other people will think he is too old.”

At one recent stop, Mr. Biden jumped from talking about the need to restore the “dignity of work” to countering Mr. Trump’s claim that he abandoned the people of Pennsylvania when he moved to Delaware with his parents as a child.

“I was 9 years old,” he said, bending over to stare directly at someone sitting in front of him, eliciting some brief chuckles. “But I guess that is how he feels about a lot of the kids coming across the border and expecting them to say, ‘Mom, leave me behind; I don’t want to be here.’ You know, anyway, I don’t want to get going on that either.”

The joke fell flat.

Miriam Kenning was left with the impression after seeing Mr. Biden at a stop in southeast Iowa that he was still finding his groove on the campaign trail. She joked that he could benefit from an occasional nap.

Ms. Kenning said Mr. Biden could provide the steady hand that voters are looking for, but that he first has to convince voters that he is excited about the chance to lead the party into the 2020 election.

“It is going to have to be an energized piece that allows that to happen for him,” the 69-year-old said. “I think he’s going to have to up the ante.”

Other voters like the Biden approach.

“He knows he can’t go crazy now because [the election] is a year and a half away,” John Wunkle said. “I mean, he’s pacing himself. I like that low-key style.”

The 71-year-old added, “I think he is going to be the nominee, and you know 77 is the new 57 — people have a lot in the tank.”

Mr. Trump faced questions about his age during the 2016 campaign but surmounted them in part with his massive rallies, which led some to liken him to a circus ringmaster.

Mr. Biden so far has come off more like a long-winded tour guide.

He bounces between stories about his parents and his personal strategy. He laments Mr. Trump’s effect on the institution of the presidency and sporadically lays out his policy plans.

And he repeatedly returns to crutch phrases. “Not a joke” is a favorite.

The meanderings also create pitfalls.

At a New York fundraiser this week, Mr. Biden fondly reminisced about two segregationists he worked with in the Senate in the 1970s and joked about their use of the racial epithet “boy” to refer to black men.

He said he was describing a more civil era in U.S. politics. Rivals, though, said he was being racially insensitive — a stinging rebuke for someone who worked beside the country’s first black president.

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