Julian Zelizer, a history and public affairs professor at Princeton University and a CNN political analyst, is author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society ." The opinions in this commentary are his own. This is the next installment in the CNN Opinion series on the challenges facing the media, under attack from critics, governments and changing technology.

(CNN) You're not going to read this in a tweet from President Donald Trump; but it can be said in a Twitter-length post: The free press is not an optional part of our democracy, it is an integral part of it. Without it, our political system can never be its best.

Yet clearly, the tensions between President Trump and the news media have heightened concern about the future of the free press. The ongoing attacks by Trump fuel public mistrust of the media, in ways that will be difficult to reverse, and create a dangerous environment where reporters might feel that it is risky to do their jobs well.

Perhaps the most troubling moment this year came when President Trump retweeted a faked video of him body-slamming and punching a man in a suit with the CNN logo plastered over his face, and many journalists trembled. As in earlier incidents, the President seemed to be intimidating and threatening the news industry in an attempt to scare them away from honest reporting.

At his rally in Phoenix this summer, the President had the crowd whipped up into a frenzy, as people yelled, "CNN sucks!" The attacks on "fake news" have been one of the President's favorite refrains.

Trump's ardent supporters see it differently: In their view, the press is out to get him. Within the Trump base, the belief is that the only reporters who can get space in the public discussion are those who are willing to be critical of the commander in chief. The press, they feel, is biased, controlled by liberals and has an agenda to attract ratings and readers at the expense of truth.

Even Harvard's Jack Goldsmith, a critic of the administration who praises much of the investigative work that's been done since January 2017, admits : "Many reporters covering Trump have overreacted and exaggerated and interjected opinion into their stories more than usual."

What often gets lost in the debate is consideration of all the ways the media's freedom enhances all of our freedoms. Here are four of them:

The watchdog

The role of the watchdog remains the most well-known function of the media -- the press has the responsibility to speak truth to power.

Beginning in 1902, Ida Tarbell published pieces in McClure's magazine, later part of her influential book, "The History of the Standard Oil Company," that revealed the ruthless business practices of John Rockefeller and his petroleum empire.

When Joseph McCarthy was incessantly attacking people as communists in the early 1950s, Edward R. Murrow broadcast a stunning episode of "See It Now" where he showed vividly how the senator fabricated accusations about fellow Americans and constantly contradicted himself. In 1969, Seymour Hersh shocked the nation with his article about how US soldiers had ruthlessly killed hundreds of civilians in the South Vietnamese village of My Lai.

When Washington Post reporters Bob Woodward and Carl Bernstein helped to break the Watergate scandal that toppled President Richard Nixon, they became a model for younger reporters entering the business. This moment in journalism was best captured when CBS correspondent Daniel Schorr, on the day that John Dean told the Senate Watergate Committee that President Nixon kept an "Enemies List," read the list live on air. "I got to No. 17," Schorr recalled, "and I said, No. 17. Daniel Schorr, a real media enemy. I almost collapsed on the air."

There is a long history of reporters since Watergate who have exposed how politicians abuse power or lie. An entire cottage industry of money and politics reporters came of age in the late 1970s and 1980s and devoted their careers to revealing the often sordid connections between campaign donations, interest groups and politicians.

During the confirmation hearings for Clarence Thomas in 1991, Nina Totenberg of NPR and Timothy Phelps of Newsday broke the story of Anita Hill's allegations of sexual harassment against the nominee. The story proved to be a watershed moment, even though the Senate confirmed Thomas, in expanding the dialogue about sexual harassment in the workplace.

Of course, no president enjoys the way that the press covers him. In this respect, Trump is not alone.

President Lyndon Johnson, pointing at the Potomac, reportedly half joked, "If I walked across the Potomac, the press would say 'LBJ can't swim.'" But as his predecessor President John F. Kennedy, who vastly expanded the number of presidential news conferences, wisely explained to NBC's Sander Vanocur: "It is never pleasant to be reading things that are not agreeable news, but I would say that it is an invaluable arm of the presidency . . . There is a terrific disadvantage not having the abrasive quality of the press applied to you daily, to an administration, even though we never like it, and even though we wish they didn't write it, and even though we disapprove."

Besides reporters working hard to expose wrongdoing and falsehood, a free press can offer written, visual and audio space for new voices to express their opinions and gain attention for issues when they are being ignored by the people in power.

This does not mean that a free press needs to be a partisan press -- in fact that tends to hurt. But rather a free press offers coverage of all perspectives, for example:

-- The anarchist Emma Goldman wrote many articles early in the 20th century questioning the rationale behind America's entrance into war and promoting women's rights to vote and to birth control.

-- The coverage of the civil rights movement in the 1960s was essential for challenging the lock that Southern Democrats had in Congress as they prevented civil rights legislation from coming up for a vote. The Rev. Martin Luther King Jr. and other activists understood the power that media coverage of white racism would have on building public pressure for legislation.

-- President Johnson's speechwriter Richard Goodwin recalls in his memoir the impact that watching the civil rights protests on television had within the White House. When anti-war activists chanted that "the whole world is watching" at the 1968 Democratic Convention, they meant that through the media the entire world could be exposed to the lies behind Vietnam.

-- During the 1970s, talk radio offered a forum for conservative voices, including Ronald Reagan, to popularize their ideas even if they did not receive sufficient attention from editors and producers.

The filter

Our political system is incredibly complicated. As the government undertook greater responsibilities in the 20th century, the thicket of federal policies and government agencies grew exponentially. The same was true at the state and local level, where government was simply doing more.

It's difficult for most Americans to make sense of everything that is happening. Our separated system of power that creates multiple arenas of governance makes this even more difficult to discern. One of the most important roles that journalists play is to make sense of all the information.

Walter Lippmann wrote that the function of the press was to "signalize an event" by bringing "to light the hidden facts, to set them in relation with each other, and make a picture of reality on which men can act."

Some journalists, like the photographer Walker Evans, have done this through vivid images of events such as the Great Depression. The war correspondent Ernie Pyle brought the country first-hand accounts of what the on-the-ground experience of soldiers was like in World War II and what the war front actually looked like beyond military talking points. Many historians credit The New York Times' Anthony Lewis for creating a cadre of journalists who specialized in the law and offered smart coverage of what was going on in the courts.

Jane Mayer of the New Yorker has been a model writer of this sort. Her work on the use of enhanced interrogation techniques during the war on terrorism, and the way that it proved ineffective, offered a model of how to help citizens sort through public policy. She did the same for campaign finance with her path-breaking work on how the Koch brothers exerted influence. Bits and pieces of what she reported were already known, but she put it all together for readers.

The sociologist Michael Schudson has explained how one of the most important developments since the 1970s was not investigative journalism, but the rise of contextual journalism, with journalists doing more work to explain the "so what" and "why" of what was taking place rather than just providing the "who, what, where and when." Great journalists have been able to offer this interpretive service to Americans.

In recent years, a younger generation of writers such as Ezra Klein, one of the founders of Vox.com, have kept up the tradition of offering digestible information about policy and politics that people can use to form perspectives of their own.

The civic arena

In the ideal situation, the press should offer a national commons where Americans can learn about and debate the great issues of the day. Even when there are many issues that divide us and many voices in newspapers and television, the goal of the press should be to create a shared environment where we understand who we are and what we need to do as a society.

One of the great costs of the current system is the immense fragmentation that Americans have in where they get their news. During the era of network television, for instance, there were only three major networks -- and a handful of major metro papers -- that commanded national attention.

As the US and the Soviet Union entered into a dangerous standoff over missiles in Cuba in 1962, the entire nation seemed to tune into the events and presidential pronouncements as people feared the confrontation could lead to nuclear war.

When a president addressed the nation, such as Lyndon Johnson's historic speech about voting rights in March 1965, anyone turning on their television would be watching the same thing.

To achieve national attention, CBS News President Fred Friendly convinced his network to temporarily switch from regular daytime programming to Sen. William Fulbright's historic hearings into the Vietnam War that triggered the first real national conversation about the rationale behind the war. Friendly's decision introduced many middle-class Americans to criticism of President Johnson's policies, but he resigned in protest when higher ups chose to air a rerun of "I Love Lucy" instead of continuing with the Fulbright hearing coverage.

When in 1986 the space shuttle Challenger tragically exploded 73 seconds into flight, killing all of the astronauts aboard, everyone tuned in to watch the aftermath and listen to President Reagan's moving remarks. Every adult can still remember the horrifying images from 9/11, when every television set, every Internet site, and every print publication offered a window to witness this attack on American soil.

Yet for the most part, in today's fragmented and individualized media environment, nothing fills the role of a central civic arena. It is difficult for anyone to capture the attention of large swaths of the public or for reporters to focus sustained national attention on critical issues.

The source of ideas, analysis and opinion

This has been a vital part of what the press can do. At the same time that reporters provide the basic information, the media has created space for smart analysts to offer opinions about what is going on. Opinion writing became a major phenomenon in the 20th century as newspapers nurtured analytical voices who were not bound by norms of objectivity.

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Writers such as the conservative icon William Buckley, founder of the National Review, helped turn publications into engines for defining and refining big ideas about the direction of politics. The goal was for the news media to do more than just report the facts.

Part of the mission of news organizations was to analyze, interpret and evaluate what was happening in the world of politics.

Although the idea of the opinion page can be traced back to the 1920s, the modern incarnation of this is usually credited to The New York Times, where in 1970 John Oakes launched the op-ed page.

The press in its best moments puts forth smart voices to offer reasoned interpretations of what is taking place and what needs to be done. There are voices who listen to what the politicians have to say and then help citizens step back and think about what their perspectives should be.

Television has embraced opinion journalism, featuring opinionated hosts and talking heads who comment on the news as much as report it. In an ideal world, journalists who occupy this space should feel free to express their arguments from all political perspectives without fear of political retribution or threat.

There are important debates taking place today as to whether opinion journalism has overtaken old-fashioned reporting to the point where it is difficult for readers and viewers to distinguish between the two.

But the problems should not cause us to overlook the important contributions that opinion journalism has made to shaping debates about politics and public policy.

The free press is not simply an institution that reports on our democracy, it is in fact a vital part of our democracy. The Trump presidency, more than anything else in recent history, has reminded us how fragile this freedom can be.