Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book " Fault Lines: A History of the United States Since 1974 ." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) To succeed in the primary and general election, former Vice President Joe Biden needs to demonstrate his willingness to form a broad coalition with the various factions of the Democratic Party.

This week, however, he once again moved in the wrong direction, drawing sharp criticism from his presidential competition. During a fundraising event in New York on Tuesday, Biden tried to highlight his ability to reach across the aisle by referencing his work in the 1970s with Mississippi Sen. James Eastland, one of the most virulent racists of the era.

"I was in a caucus with James O. Eastland. He never called me 'boy,' he always called me 'son,' " Biden said. "Well guess what? At least there was some civility. We got things done," he said

But Eastland, a well-known segregationist who spent most of his life defending white supremacy, was one of the southern Democrats responsible for holding back bills that would have integrated the South and provided voting rights for African-Americans. Biden's decision to invoke Eastland became even more damaging when archival research resurfaced , showing how Biden sought Eastland's help on anti-busing legislation in 1977.

Liberal critics rightly jumped at Biden's choice and questioned why he didn't highlight his work with less notorious southern Democrats or Republican Sens. Robert Dole or John McCain instead. Given how devastating the power of southern Democrats had been to civil rights during the 1950s and 1960s, there was no reason -- especially in 2019 -- to fondly recall any kind of working relationship with Eastland.

Biden launched his campaign in April with a video emphasizing the values at stake in this election. To drive home the point, he said there was a "battle for the soul of this nation" and recalled President Donald Trump's infamous response that there were "very fine people on both sides" at the far-right rally in Charlottesville in 2017. Given this context, liberals argue Biden should be much more sensitive to why his comments about Eastland matter. While civil rights icon Rep. John Lewis came to Biden's defense , other Democrats, including presidential hopeful Cory Booker called for an apology. Sen. Kamala Harris, another 2020 candidate, said Biden's comments were "deeply wrong."

JUST WATCHED GOP panelist: Biden's comments on segregationist senators illustrate why he won't be the nominee Replay More Videos ... MUST WATCH GOP panelist: Biden's comments on segregationist senators illustrate why he won't be the nominee 06:11

This is not the first time that Biden has clashed with liberal voices in his party. Other fault lines have already emerged on his changing positions toward reproductive rights (his support of of the Hyde Amendment that prohibited federal funds from being used for abortion), sexual harassment (his treatment of Anita Hill who accused Clarence Thomas sexual misconduct) and crime (his sponsorship of the 1994 crime legislation and its aftermath). With the exception of his reversal on the Hyde Amendment, Biden has been pretty resolute in defending his positions.

Conservatives are loving the turmoil. Bret Stephens of The New York Times predictably jumped into the fray , dismissing the "nasty left" by calling their approach to politics "apocalyptic."

Stephens goes so far as to compare the left to the Trumpian right. Repeating his common "both sides do it" refrain, Stephens posits that liberals who take issue with Biden's willing associations with a rabid white supremacist are just as extreme as Trumpian Republicans. Stephens goes on to suggest that Biden should disregard the left's criticism, writing that he would be "a fool" to apologize for his remarks.

Unfortunately, Biden and his team seem to be responding accordingly. He has often refused to engage with the criticism from his own party or in some cases, pushed back. In one particularly dramatic moment this week, Biden responded to Booker's call to apologize by saying, "Apologize for what? Cory (Booker) should apologize. He knows better. There's not a racist bone in my body."

Biden's approach is curious and flawed. Given that much of his campaign has been built around the idea that he is the candidate best suited to reach out to Independents and Republicans, why is he displaying such a different attitude when it comes to members of his own party? Rather than showcasing the civility he keeps talking about with Republicans, he has taken a much more adversarial stance with liberals.

It begs the question that if he can't make peace with people in his own party who are generally on the same page in terms of values and policies, how will he ever convince Republicans like Sen. Mitch McConnell to cooperate with him on anything? Shouldn't aggressive outreach start at home? To defeat Trump, an incumbent with a strong economy and solid party support, Biden will need the full force of his party to mobilize supporters and turn out the vote needed to win.

Biden needs to spend more time between now and January demonstrating that he can be a coalition builder within the Democratic Party. He needs to take seriously the concerns that have arisen about his long record in the Senate as well as his recent fumbles by addressing his critics rather than waving them away. While some conservatives often dismiss the disproportionate noise of "woke" progressives, Biden should take stock of the party, and take the time to craft speeches and policy proposals that will demonstrate an ability to bridge the Democratic factions.

Effective candidates like Bill Clinton and Barack Obama were able to rally the party together, even during the course of heated primaries. Clinton used the recession to do so, while Obama did it with his opposition to the war in Iraq. If Biden really wants to thrive during the primaries, he should stick to the message laid out early in his campaign launch and focus on the threat of a second term for Trump.

Let's not forget -- while Biden's ability to work with Republicans might seem like a pleasant talking point, there's no evidence it would produce results in today's fevered political world. Given the partisanship that has taken root in the decades since Biden first entered Congress, harping on about reaching across the aisle may appear out of touch with reality.

If Biden wants to be the Democrat his supporters believe he can be, he needs to ignore the Bret Stephens of the world and instead show how that he can be a coalition builder among Democrats unlike anyone else in the field. It's time for Biden to demonstrate the same willingness to reach out to his fellow Democrats as he does with right-wing Republicans who have little interest in doing anything other than biting off his outreached hand.

Stay up to date... Sign up for our new newsletter. Join us on Twitter and Facebook

At this pivotal moment in the campaign, Biden might recall different words from John Lewis, who stood before civil rights activists at the March on Washington in 1963 and warned of the high price parties pay as a result of racism.

Lewis said , "My friends, let us not forget that we are involved in a serious social revolution. By and large, American politics is dominated by politicians who build their careers on immoral compromises and ally themselves with open forms of political, economic, and social exploitation. There are exceptions, of course. We salute those. But what political leader can stand up and say, 'My party is the party of principles?' For the party of Kennedy is also the party of Eastland. The party of Javits is also the party of Goldwater. Where is our party? Where is the political party that will make it unnecessary to march on Washington? Where is the political party that will make it unnecessary to march in the streets of Birmingham."

The social revolution remains incomplete. Biden, who served in the historic administration of the first African-American president, has the opportunity to campaign as a Democrat who will heed Lewis's warning many decades later. Doing so will require recognizing the dangers of compromise on certain key issues and dealing head-on with the racism that remains a blight on our country.