Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter," published by Times Books, and editor of a book assessing former President George W. Bush's administration, published by Princeton University Press.

(CNN) -- At key moments in his presidency, Barack Obama has struggled to win the support of the American people through the power of his oratory. The power of persuasion has traditionally been one of the most powerful weapons of the commander in chief.

On Wednesday, in his most recent effort to recreate the power of the bully pulpit in the modern age of communication, the president conducted a town hall tweet session on the White House Twitter account, which enjoys more than 2 million followers. This is the latest effort in a series of attempts to use social media, such his Facebook town hall, to communicate to the American people.

Thus far, however, President Obama has had trouble taking his case directly to the public. This is ironic and puzzling since he was the candidate who dazzled Americans during the 2008 campaign, including many Republicans.

How can this be? Throughout the 20th century, presidents used the bully pulpit to win public support. President Theodore Roosevelt courted reporters and delivered major speeches, covered by most of the newspapers, on key legislation such as railroad regulation and food inspection.

During the 1930s, President Franklin Roosevelt used the radio to powerful effect, tackling the Great Depression through communication. Roosevelt went on the air when he wanted to sell a certain policy or calm Americans about a particular challenge. "Until Mr. Roosevelt taught the world how that titanic trombone of tubes and antennae could be played," noted Fortune magazine, "no one had any idea of the possible range of its virtuosity."

When President Harry Truman wanted to sell his anti-communist policies, he also relied on the power of a presidential message. Truman wanted to persuade Congress to provide assistance to Greece and Turkey, and to sell the Truman Doctrine through which the U.S. would commit to supporting anti-communist forces around the world. Republican Sen. Arthur Vandenberg advised the president to "scare the hell out of the American people." And that he did. Truman delivered a speech on March 12, 1947, that marshaled support for the specific request and built enthusiasm for his broader policies against the Soviet Union.

The pattern continued throughout the remainder of the century. At the height of the Cuban Missile Crisis in October 1962, more than 50 million Americans turned on their televisions to watch President John F. Kennedy explain the quarantine the United States was imposing around Cuba to prevent Soviet ships from bringing weaponry onto the island. The address had a powerful effect. The New York Herald Tribune, a conservative paper, proclaimed that "The people of the United States must and will unite behind the president in the course which Soviet aggression has made inevitable."

After police attacked voting rights protesters in Alabama in March 1965, President Lyndon Johnson appealed to Congress and the nation to pass voting rights legislation. He went on television and went so far as to say, "We shall overcome," using the words of the civil rights movement as his own. Congress swiftly passed the measure.

President Ronald Reagan was one of the last presidents to govern in an era when presidents could really make their case. When he wanted to sell his tax cut in 1981, he went to the American public, armed with charts and statistics to explain why tax cuts could lower deficits and boost the economy.

When he wanted to build support for his increased military budget, Reagan made a number of well-publicized speeches warning of the danger of communism and the need for Americans to be tough. And when Reagan thawed, and made peace with the Soviet Union, he used some well-calibrated events and talks to overcome resistance.

But the current structure of the media has emasculated the bully pulpit. Regardless of how good a president is on the stump, it is almost impossible for him to command public attention, because there is no singular "media" to speak of. Instead, Americans receive their media through countless television stations and websites.

During the 1960s, when Presidents Kennedy, Johnson or Nixon spoke, the choice was to hear them or turn off the television and radio. Today, if President Obama wanted to conduct a fireside chat, it is doubtful that many people would be listening.

With the end of the Fairness Doctrine in 1987, the media were also able to shed the appearance of neutrality and objectivity. Every perspective did not have to receive equal time. On many television and radio stations, objective reporters have been replaced with openly partisan commentators. Any presidential message is quickly surrounded by polemical instant commentary that diminishes the power of what he says.

Making matters worse, on the Internet, presidents can't even fully control the time they have as they must compete with live blogs and video commentary as they try to share their message. Even within most households, the era of the single family television is gone. Now in many middle-class families everyone has their own media and is watching their own thing.

President Obama has gone to great lengths to find new ways to reach the American people. But he is trying to achieve a 20th-century goal in a century when it is no longer possible. The reality is that presidents, Democrat or Republican, will have to find new ways to exercise what power they have and should no longer expect the opportunity to simply take their case to the public.

The opinions expressed in this commentary are solely those of Julian E. Zelizer.