Editor’s Note: 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.” The opinions expressed in this commentary are his own. View more opinion at CNN.

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Congresswoman Ilhan Omar took a shot against former President Barack Obama. During an interview with Politico, the controversial congresswoman was critical of Obama for working within a broken system, pointing to his immigration and drone policies as examples of when Democrats in the past made huge errors that created the path to the problems of today.

Omar was quoted as saying: “We can’t be only upset with Trump. … His policies are bad, but many of the people who came before him also had really bad policies. They just were more polished than he was.” She also said, “And that’s not what we should be looking for anymore. We don’t want anybody to get away with murder because they are polished. We want to recognize the actual policies that are behind the pretty face and the smile.”

Importantly, Omar did distinguish what President Trump has done from his predecessor. Everything is not the same. But her bigger point is a familiar argument that we have heard from the left, including from Bernie Sanders in 2016, that unless there are structural changes in public policy and the organization of government, the differences that will result from one party or the other controlling the branches of government will be limited.

Having younger members criticize party elders is not new, nor is it always a bad thing. Throughout American history, generational change within Congress has produced fresh voices who are willing to say tough things about revered senior party leaders. President Franklin Roosevelt came under fire from liberals who thought he didn’t go far enough to reform capitalism, while civil rights advocates often felt that President Lyndon Johnson was too timid on racial justice and too invested in a bad war in Vietnam. This kind of criticism, no matter how unpleasant, can have beneficial effects by pushing new ideas that make the party stronger and, if successful, help the nation improve.

But historically, there is always a danger that the left goes too far in flattening any differences between its own party and its opponents. This was the kind of thinking that produced support for Ralph Nader’s third-party campaign in 2000. “The only difference between Al Gore and George W. Bush is the velocity with which their knees hit the floor when corporations knock,” Nader said during a stop in the 2000 campaign. The danger of this logic is that the mavericks unintentionally dampen the enthusiasm of younger voters whose energy and ideas will be essential to victory. This “Tweedledee” and “Tweedledum” world view can cause some to miss the fundamental issues that are at stake.

Worlds apart

President Obama and President Trump are worlds apart despite any continuities that exist.

On domestic policy, President Obama left behind a sizable domestic record that moved the social contract in a progressive direction, whereas Trump has generally focused on cutting domestic spending and retrenching regulations aimed to protect middle-class workers, Obama had a very different agenda.

The Affordable Care Act and the economic stimulus greatly expanded federal benefits for millions of Americans, and the stimulus helped moved the economy out of a terrible recession and into the current era of economic growth. The financial regulations, which were far from perfect, instituted new restraints on excessive risk-taking and offered protections for middle-class investors.

While President Trump has spent much of his time in office stripping away protections for people who tried to enter our country from overseas and curbing climate change, President Obama created DACA to protect the children of adults who arrived without documentation and put into place regulations to reduce carbon emissions. Yes, President Obama implemented extremely tough border policies, but he was pursuing a grand bargain with the GOP that would include a path to citizenship for millions of people.

Though far short of the goals of Black Lives Matters, the Obama administration’s Attorney General Eric Holder implemented important reforms that lessened some of the worst racial disparities in the criminal justice system. The Department of Justice, for instance, ended mandatory sentences on low-level offenders. The cumulative effects of Obama’s domestic programs, Princeton University sociologist Paul Starr reminds us, diminished economic inequality.

On foreign policy, President Obama was far from perfect. The left will never forgive Obama for accelerating the use of targeted drone attacks to assassinate terrorists. His policy in Syria turned out to be problematic, worsening the instability in the region. While he did fulfill his campaign promise to draw down forces in Iraq and Afghanistan, his critics believe he left behind too much instability.

But overall, he made immense gains in strengthening America’s standing overseas and restoring international alliances, like NATO. Favorable perceptions of the United States more than doubled in Germany during the time of his presidency, while confidence in the US president in the United Kingdom rose from 16% for Bush in 2008 to 86% to Obama in 2009.

Obama was determined to work international alliances whenever possible. He was a believer in the promise of post-World War II America that through partnerships we could achieve more than by going it alone. Obama entered into the Paris Agreement on climate, the Trans-Pacific Partnership and the Iran nuclear deal, all of which leveraged international alliances to curb global dangers. These are the very treaties that President Trump has torn apart.

The Obama administration’s drone attacks and counterintelligence operations did significantly damage the infrastructure of al Qaeda and ISIS. While Obama certainly did not undo President George W. Bush’s counterterrorism apparatus – expanding, for instance, surveillance – he did end the use of torture and implemented reforms that fixed some of the problems that had been exposed.

In terms of their style of leadership, Obama and Trump couldn’t be further apart. Obama privileged pragmatic, rational and deliberative decision-making, refusing to be sucked into the frenzied and chaotic media environment. Even though he lived in a fiercely partisan world, for better or worse he refused to play by those rules.

Although he frustrated Democrats who wanted more of a fight, he was the polar opposite in terms of how he conceived of leadership compared with our current President. He was careful with his words and respectful of norms. He continued to try to appeal to Republicans, even though the vast majority were not going to agree with him. He also ran an administration that was generally free of corruption scandals.

Appealing to hope

In both of his campaigns (2008 and 2012), Obama ran an operation that appealed to the most optimistic and hopeful elements of the American character. In 2008, Obama called for Americans to change the way that their politics worked, and he refused to accept the idea that the nation was sliced and diced into red and blue. He underestimated the intensity of the polarization but at least tried to bring out the best of the country. President Trump’s preference has always been to divide, attack and scare.

President Obama sought a coalition that thrived as a result of its diversity – a coalition of African-Americans, younger Americans, suburban educated women and independents – as opposed to President Trump, who is doubling down on the rural white vote.

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    There is a great deal for Democrats to criticize about Obama. As the editor of a book about his presidency, I have spent a good deal of time pointing out how his failure to strengthen the Democratic Party helped create the conditions that allowed for President Trump’s shocking victory in 2016. He underestimated the extremism that had taken hold of the Republican leadership, failing to push back hard enough as the party veered sharply to the right.

    Like Sanders, Omar points to difficult aspects of the Obama legacy – such as the significant increase in deportations and the failure to prosecute Wall Street tycoons for the crash of 2008 – that don’t sit well with much of the party’s base.

    But even when criticizing and highlighting continuities, it is important to understand the fundamental differences that separate Presidents Obama and Trump.

    The reasons this matters is not simply to understand Obama’s legacy but to shape the way Democrats campaign in 2020. The party needs to communicate the fundamental differences between the Trump administration and any Democratic president who will follow.

    Given that the Republican leadership is now in line with the extremist elements in the conservative electorate, Democrats should be clear that moderate Democrats are in a different universe than the top figures in the GOP.

    Getting caught up in intra-party fighting over policy differences would be a mistake. The party’s main message has to remain on target: to turn out and to vote, regardless of whether its preferred candidate ends up on top in the primaries. In the end, the energy that would drive the party toward victory will revolve around Democrats making very clear to the electorate what a fundamental difference it would make in terms of policy, politics and leadership for Trump’s presidency to come to an end.