Julian Zelizer, a history and public affairs professor at Princeton University and a CNN political analyst, is the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society ." He's co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own.

(CNN) Fired FBI Director James Comey will be the center of the political world's attention Thursday when he testifies before the Senate Intelligence Committee.

It could be a historic moment for Donald Trump and the American presidency and one that ranks with a number of other times testimony before Congress has brought about big political shifts.

Since the day that the President abruptly fired Comey , we have heard allegations from sources familiar with the matter that the President was pressuring him to stop the investigation into possible collusion between Russians and members of the Trump campaign.

If the former FBI director appears before the Congress and says, without hesitation and with evidence, that the President was trying to obstruct the investigation, then the White House has a very serious problem on its hands and we would move closer to the possibility of impeachment proceedings.

We have not yet heard from Comey himself and it is hard to know exactly what he will say. The testimony could turn into a bust for President Trump's opponents and a boon to the administration if Comey doesn't say anything of interest. But it's instructive to look at how other famous witnesses have changed history.

Here are a six of the most important moments in chronological order.

Joseph Welch (1954): "Have you no sense of decency?"

Joseph N. Welch, during the Army-McCarthy hearings.

This was a classic moment where someone used the hearings to stand up to the political crusade being led by Sen. Joseph R. McCarthy that alleged communist infiltration of the US military. Joseph Welch , the special counsel of the Army, attacked the senator for accusing a young lawyer from his firm of having been part of a left-wing group while in law school. "Senator, may we not drop this? We know he belonged to the Lawyer's Guild," Welch said to interrupt the questioning.

JUST WATCHED 1950: McCarthy claims communist infiltration Replay More Videos ... MUST WATCH 1950: McCarthy claims communist infiltration 02:39

The senator did not cease nor did he desist. "You've done enough," Welch said before the television cameras, expressing the sense of frustration that many in Washington had with McCarthy's vicious anti-communist crusade, "Have you no sense of decency, sir, at long last? Have you left no sense of decency?"

This had been a choreographed moment to bring down Sen. McCarthy. Welch's comment proved to be a turning point after which more people in Washington were willing to stand up to the ruthless senator and his allies.

George Kennan (1966): "Vietnam is not a region of major military, industrial importance"

Former Ambassador George F. Kennan testifies about the Vietnam War.

Sen. William Fulbright, a liberal internationalist from Arkansas, helped President Lyndon B. Johnson obtain backing for the Gulf of Tonkin Resolution in 1964 , which granted broad authority for military intervention in Southeast Asia.

Two years later, Sen. Fulbright had turned against the war and believed that it was becoming a disaster. He convened televised hearings into the war. Americans watched as high level diplomatic officials discussed their problems with US involvement in the conflict.

Former Ambassador George F. Kennan, the well-respected diplomat who was an architect of the containment policy, admitted that, "If we were not already involved as we are today in Vietnam, I would know of no reason why we should wish to become so involved, and I could think of several reasons why we should wish not to. Vietnam is not a region of major military, industrial importance."

These words, coming from a prominent foreign policy figure rather than a college protester, had a big effect on Americans, who until then had confidence in President Johnson's promises of victory. While support for the war remained strong, the testimony and the rest of the hearings opened up a discussion about the legitimacy and need for the war in middle-class America that would continue for years to come.

John Dean (1973): "I began by telling the president that there was a cancer growing on the presidency."

Former White House counsel John Dean testifies before the Senate Watergate committee.

This remains the gold standard for game-changing testimony at congressional hearings. As a counsel to President Richard Nixon, John Dean appeared before Sen. Sam Ervin's Watergate committee . He shared with the nation that he had frequently discussed ways through which the White House could stop the investigation of the president.

Dean claimed that there were conversations about payments to buy the silence of potential witnesses as well as promises of executive clemency for those who stood by the administration. He said, "I began by telling the president there was a cancer growing on the presidency, and if the cancer was not removed, the President himself would be killed by it."

JUST WATCHED Dean: Trump's behavior similar to Nixon's Replay More Videos ... MUST WATCH Dean: Trump's behavior similar to Nixon's 01:30

"If Dean's claims are true -- and his supporting details as well as some of his circumstantial documents were impressive -- that would make Nixon's May 22 denials outright lies or at least render the presidential statements once again 'inoperative,'" noted Time magazine

Dean's testimony lasted for almost five hours. Once Dean finished, the future of the presidency was on thin ice.

William Colby (1975): "It is a very deadly weapon"

Committee Chairman Frank Church displays a poison dart gun during testimony about the CIA.

In the aftermath of the Vietnam War, Idaho Sen. Frank Church conducted high-profile hearings into the CIA with the goal of exposing the wrongdoing that had been committed by the agency in the name of national security.

One of the most shocking moments of all came when CIA Director William Colby, who had assumed his position just two years earlier, admitted to the "dirty tricks" in which his agency had engaged. Colby, for instance, explained "M.K. Naomi," a top-secret project in which the agency developed poisons, biochemical weapons and electric dart guns that could fire darts filled with a lethal shellfish toxin over 100 meters ( Church held one up before the cameras for dramatic effect).

Colby admitted that the dart guns was a "very deadly weapon." The testimony offered considerable fodder for the growing number of left-wing critics of the agency, and undermined his own standing as director. President Gerald Ford would relieve Colby of his position, believing that he had spoken too much about the CIA's internal list of misdeeds that were considered illegal, unethical, or inappropriate. Investigative reporter Seymour Hersh exposed these acts in 1974; they had taken place between the 1950s and the mid-1970s.

Lt. Col. Oliver North (1987): "I came here to tell you the truth, the good, the bad, and the ugly. I am here to tell it all, pleasant and unpleasant. And I am here to accept responsibility for that which I did. I will not accept the responsibility for that which I did not do."

JUST WATCHED 1986: Oliver North and the Iran-Contra scandal Replay More Videos ... MUST WATCH 1986: Oliver North and the Iran-Contra scandal 01:43

This was a case where the congressional hearings moved in a very different direction than the opposition party expected. As Democrats called Lt. Col. Oliver North, the point man in the Iran-Contra scandal to testify, the reaction was different than they thought.

The public loved North, who, dressed in military garb and stood by his decision to provide secret assistance to the Nicaraguan Contras despite a congressional ban against doing so. Television audiences loved him because he was a welcome antidote to the defeatist attitudes of the post-Vietnam era.

His defiant stance strengthened support for President Ronald Reagan, who had been struggling ever since the scandal broke. Reporters noted that " Olliemania " swept the country with many citizens looking at him like a modern-day hero. He was on the cover of weekly magazines, and on T-shirts and buttons, dolls and food (the Ollie-sandwich in Buffalo, New York featured red-blooded American beef on a hero roll with shredded lettuce), videotapes and books -- and there were even cardboard pictures throughout many cities that people could stand alongside so their pictures could be taken.

This could have been a moment where the investigation deteriorated, but instead it was the start of a rebound. The obsession with North faded fast (unsold North dolls were made into Gorbachev dolls a few months later), but the positive impact for Reagan was long-lasting.

Tobacco executives (1994): "Do you understand how isolated you are from the scientific community in your belief?"

Tobacco executives are sworn in before congressional hearing.

This was a case in which the testimony that Americans watched was so unbelievable that it discredited the cause of the witnesses. Seven top executives came to Washington as Congress was trying to reach a decision on how to regulate tobacco

All of the executives, sitting side by side at a conference table, shockingly testified that tobacco was not addictive.

When they only admitted that cigarettes "may" cause lung cancer and heart disease, California Rep. Henry A. Waxman blasted all of the executives for being so out of step with the medical community. "Do you understand how isolated you are from the scientific community in your belief?" "I do sir," admitted Andrew H. Tisch, the chief executive of the Lorillard Tobacco Company.

Follow CNN Opinion Join us on Twitter and Facebook

The appearance was disastrous for the tobacco industry , though a boon to public health. The executives also made admissions under pressure, such as confirming that the companies controlled the amount of nicotine in cigarettes and a number of their brands gave smokers higher than normal nicotine.

James W. Johnston, the chairman of R.J. Reynolds, apologized for the advertisements with Joe Camel that appealed to younger men and which featured images of men engaging in aggressive behavior toward women. "It never should have run. I apologize," he said. After the hearings, public pressure grew for a stronger regulatory program and lawsuits to try to curb smoking.