Former vice president Joe Biden faced a fourth day of criticism Sunday over his long-held opposition to busing black children to majority-white schools to desegregate them, highlighting the difficulty his campaign has had moving beyond an issue that erupted during Thursday’s debate.

Sen. Cory Booker (D-N.J.) called out Biden on “Meet the Press” on Sunday, saying he has failed at “bringing people together” on racial issues because he won’t speak “candidly” about past legislative work, such as his support for antibusing legislation and work on the 1994 crime bill, which contributed to mass incarceration.

“He’s causing a lot of frustration and even pain with his words,” Booker said. “When it comes to difficult issues with race, if you can’t talk openly and honestly about your own development on these issues, I think it’s very hard to lead our country forward.”

Race has become a major flash point of the Democratic presidential campaign, and Biden’s repeated stumbles raise questions about whether he’ll be able to pivot from a messy discussion of his lengthy record to a positive message about how he would govern.

Faced with sustained attacks from the two prominent black candidates in the Democratic field, Biden has pointed to his record supporting civil rights legislation and his work with President Obama, the country’s first black president. He is also planning to release a criminal justice plan and has met with some members of the Congressional Black Caucus to garner support.

In a statement Sunday, Jamal Brown, a Biden spokesman, said that “Joe Biden is today — and has been for more than 40 years in public life — one of the strongest and most powerful voices for civil rights in America.” He added that Biden “has worked to advance legal and social progress for African Americans and defeat systemic racism and unacceptable racial disparities.”

But Biden’s struggles have persisted since mid-June when, during a fundraiser in New York City, he bragged about his ability to work with two segregationist senators. At the time, Booker demanded that Biden apologize. Biden doubled down, saying he was the one owed an apology.

In the sharpest exchange of the first Democratic debate, Sen. Kamala D. Harris (D-Calif.) said Biden’s comments were “hurtful.” She also drew attention to Biden’s history of opposing busing. “I was part of the second class to integrate Berkeley, California, public schools almost two decades after Brown v. Board of Education,” Harris said.

On Sunday, spokesmen for Biden’s and Harris’s campaigns sparred over the topic via Twitter.

“Busing is sometimes appropriate,” tweeted TJ Ducklo, a Biden spokesman, adding that Biden supports “voluntary” busing, which he said was how Harris’s school was desegregated.

Ducklo argued that Biden supports “better policies [that] address the root causes of segregation,” including rezoning school districts, placing public housing in white neighborhoods and combating redlining.

“With all due respect to you and VP Biden, this isn’t true,” Ian Sams, a Harris spokesman, shot back. “There was no voluntary vs. forced distinction.”

Sams invoked comments made by Biden in the mid-1970s against busing. “Who the hell do we think we are, that the only way a black man or woman can learn is if they rub shoulders with my white child,” Biden said at the time.

Biden also caught flak for comments he made about black teenagers Friday at an event for Rev. Jesse Jackson’s Rainbow PUSH Coalition in Chicago.

“We’ve got to recognize that kid wearing a hoodie may very well be the next poet laureate and not a gangbanger,” Biden said, in an attempt to express his desire to eradicate racism.

That fell flat with opponents. “This isn’t about a hoodie. It’s about a culture that sees a problem with a kid wearing a hoodie in the first place,” Booker tweeted.

The Biden camp also pointed to comments made Sunday by Sen. Bernie Sanders (I-Vt.) on busing, in an attempt to show that others in the field take issue with the practice.

“Busing is certainly an option that is necessary in certain cases, but it is not the optimal,” Sanders said on ABC’s “This Week.”

“Does anybody think it’s a good idea to put a kid on a bus, travel an hour to another school, into another neighborhood that he or she doesn’t know?” he said. “That’s not the optimal. What is the optimal is to have great community schools, which are integrated.”

On Saturday, Biden stumbled in a different area: gay rights. At a fundraiser in Seattle, he said public sentiment toward gays and lesbians has changed significantly in recent years, and five years ago, if someone at a business meeting in Seattle “made fun of a gay waiter,” people would just let it go.

The audience vocally pushed back at that, saying “Not in Seattle!”