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.

(CNN) 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.