By Jonathan Martin and Katie Glueck | New York Times

Former Vice President Joseph R. Biden Jr. marched in a parade in Independence, Iowa, on Thursday. Jordan Gale for The New York Times

INDEPENDENCE, Iowa — Joseph R. Biden Jr. expressed frustration Thursday at Senator Kamala Harris’s pointed criticism about his 1970s-era opposition to busing, arguing that Democrats should “be talking about the future.” But he resisted the opportunity to return fire at Ms. Harris for voicing a position similar to his on school integration.

One day after she said local school districts should determine whether to bus students, effectively the argument Mr. Biden had made in the face of Ms. Harris’s attack at a debate last week, the former vice president simply said that she was “absolutely right.”

As he addressed reporters after jogging through a July 4 parade on a steamy morning in Independence, Iowa, Mr. Biden called Ms. Harris, who has jumped in the polls since her debate performance, “a good person, smart as she can be.”

He also repeatedly made clear that he was irritated at being targeted for positions he had held more than four decades ago.

“This is kind of a new thing, you know we’re going back 40 or 50 years now to a vote,” he said, vowing that he would not take on his rivals for the Democratic presidential nomination in a similar way. “I’m not going to go back and talk about the record of anyone from 10, 20, 30 years ago,” Mr. Biden said, adding that “everything is lost in context.”

Ms. Harris has surged with African-American voters, a crucial constituency for Mr. Biden, since her performance at the debate. He bridled at the suggestion that he should show contrition for opposing federally mandated busing — he once called the approach “asinine” — during his tenure as a senator from Delaware. “I don’t have to atone,” he said. “Look, my record stands for itself.”

Mr. Biden made the case to “move on and talk about what we do now,” but he may have handed new fodder to his critics on the left. His comments minimized busing, an issue that still lingers for many Americans who experienced the mandated integration practice or whose children attend schools that are de facto segregated by race.

“Busing is something 99 percent of the American people don’t even know what we’re talking about,” he argued.

Senator Kamala Harris spoke with voters at a Fourth of July house party in Indianola, Iowa, on Thursday. Hilary Swift for The New York Times 

Mr. Biden said he was satisfied with his debate performance, which was widely criticized. He dismissed several polls taken in the aftermath of the back-to-back forums that show his once-solid lead evaporating.

“I’m still way ahead,” he said.

Mr. Biden is not the only candidate struggling with how to talk about school integration. After Ms. Harris found success confronting the former vice president on busing, she has appeared uncertain how to characterize her own views on the matter.

Before addressing a July 4 house party in a sweltering backyard in Indianola, Iowa, Ms. Harris insisted that she and Mr. Biden did not share the same position on federally mandated busing. She then attempted to focus the conversation in the past rather than the present.

“I have asked him and have yet to hear him agree that busing that was court-ordered and mandated in most places and in that era in which I was bused, was necessary,” Ms. Harris said of her childhood in Berkeley, Calif., in the late 1960s and 1970s. “He has yet to agree that his position on this, which was to work with segregationists and oppose busing, was wrong.”

Ms. Harris was asked to explain what she meant when she said Wednesday that busing should be part of “the toolbox” to address desegregating schools, which would be distinct from a federal mandate. She suggested that the environment around civil rights is different today from when she was a student, though she said she supported school districts and municipalities doing “whatever they need to do” on integration measures.

This is not the first issue on which Ms. Harris has muddled her response — she has also struggled to articulate whether she thinks private health care should be eliminated — but the California senator dismissed questions about her consistency.

“I have not changed my position,” she said. “So, we can talk about other issues if you’d like.”

After Ms. Harris took questions from reporters, she received an enthusiastic reception as she addressed a crowd from a back porch overlooking a backyard and Iowa fields.

Several voters shrugged off the busing controversies, suggesting that it was not a major issue as they assessed the race.

“I feel everyone has something in their past,” said Ashley Raske, an African-American woman from Des Moines who said she was considering Ms. Harris and Mr. Biden as top choices. “I’m looking a lot at what they stand for now, how they’re going to serve my family and the American people.”

But the exchanges around busing prolonged the most combative period of the Democratic primary to date. In the days after the debate, several of Mr. Biden’s allies and aides said privately that they were surprised by Ms. Harris’s criticisms of Mr. Biden’s civil rights record. Some even said they found her words hurtful.

Senator Bernie Sanders of Vermont marched in an Independence Day parade in Ames, Iowa. Hilary Swift for The New York Times

On Thursday, Ms. Harris said that Mr. Biden could not have been too surprised to be questioned about his record after his praise last month for a Senate that included notorious segregationists.

“That was the subject of conversation for days at end,” she said. “So you know if he and his team weren’t prepared for the topic I don’t know what to say about that.”

But Ms. Harris plainly does not want to revive the issue as a matter of policy today.

Returning to Iowa for the first time since she vaulted into the top tier of the race, and made no mention of the former vice president in her stump speech, instead unveiling a new line of attack on President Trump.

“We have a predator living in the White House,” she told Democrats in Indianola.

Mr. Biden was similarly disinclined to focus on the issue on Thursday. After the parade, he delivered an Independence Day-themed speech in Marshalltown. Reading from prepared remarks in a teleprompter, Mr. Biden hailed America, summoned the words of past presidents and flayed the incumbent.

Recalling that President Vladimir V. Putin of Russia had crowed that Western liberalism was becoming obsolete, Mr. Biden prompted groans in a friendly crowd when he noted that Mr. Trump thought Mr. Putin was alluding to California-style progressive politics.

“Not a joke,” Mr. Biden said, “our president doesn’t understand the difference between liberals as opposed to conservatives in our political context and liberal as opposed to autocratic systems of government.”

Mr. Biden and Ms. Harris were hardly the only Democrats in Iowa for July 4. Former Representative Beto O’Rourke of Texas joined Mr. Biden for the parade in Independence, which was filled with fire trucks, beauty queens and a few supporters of Mr. Trump who shouted their enthusiasm for the president when volunteers for Democrats chanted their candidates’ names.

Biden supporters distributed Tootsie Rolls and Dubble Bubble, and Mr. Biden sweated through his polo shirt as he wandered from side to side of a parade in the blue-collar eastern Iowa village.

When he eyed three women sporting Biden lapel stickers and standing in the bed of a truck on the side of the parade route, he jokingly yelled: “Don’t jump, I need you!” That was shortly before he cradled a 4-month-old baby and pretended to walk away from the infant’s mother.

After the parade, as Mr. Biden soberly addressed questions about integration and race in America, he could not fully escape the holiday’s festivities.

The holiday parades in Iowa were a sharp contrast to the one unfolding in Washington, where President Trump had ordered up a military-style event complete with tanks and fighter jets. Hilary Swift for The New York Times

“I’ve always supported voluntary busing,” he explained, moments before the sound of a teenage girl’s voice pierced the air. “It’s Joe Biden!” she said.

Senator Bernie Sanders of Vermont also hit the parade circuit, beginning his day in Slater, Iowa, about half an hour from Des Moines.

The scene was a sharp contrast to the one unfolding in Washington, where Mr. Trump had ordered up a military-style event complete with tanks and fighter jets.

In Slater, there was a yellow truck representing the Story County Democrats, bearing a multicolored sign reading “love is love.” Girl Scouts congregated around a pickup truck.

Mr. Sanders arrived at the parade in sneakers and walked through the town and leafy side streets, waving and belting out, “Hello!”

He occasionally took pictures or shook hands, but generally stayed in the middle of the street as more than two dozen activists and people associated with his campaign followed behind, chanting progressive slogans such as “We don’t need no super PAC! Bernie Sanders got our back!”

One man, who declined to give his name to a reporter, insisted that his young daughter remove a Sanders sticker she had received.

“I’ll give her 100 stickers if she takes it off,” he said. “And I’ll pay for it myself with my own money I earn.”

In Ames, Iowa, home to Iowa State University, Mr. Sanders got a more robust reception as he marched down Main Street, which was dotted with American flags.

He moved rapidly through the parade, stopping to take pictures, though he clearly did not want to linger. “Very quickly,” he said, in response to a selfie request.