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) During the 2016 campaign, Donald Trump famously told the world that he could "stand in the middle of Fifth Avenue and "shoot somebody" and he wouldn't "lose any voters."

For some time, his prediction has appeared to be true. No matter what he said and regardless of what he did, the famous "base" continued to express their support for him. Most Republican legislators and voters have dismissed the Russia scandal as a phony distraction by the media.

Even as the President bungles the Republican moment of united government with a huge legislative nothing-burger and has national approval polls that have fallen from 42 percent in April to a political danger zone of 36 percent, they have swallowed their pride as they try to make the most of the time that is left before everyone turns their attention to the midterms.

But this week something changed following the revelation of Donald Trump Jr.'s June 9, 2016 meeting in Trump Tower. The release of Trump Jr.'s emails, the discovery of the meeting and the frequently changing account of what the meeting was about and who was there has shaken the confidence of a key player in the world of politics -- the conservative media.

Over the past week, a number of conservative commentators have expressed some pretty strong criticism of President Trump. In his column, entitled "A Conspiracy of Dunces," the conservative New York Times columnist Ross Douthat began by saying that, "Here is a good rule of thumb for dealing with Donald Trump: Everyone who gives him the benefit of the doubt eventually regrets it" and also wrote that "we should drop the presumption that such collusion is an extreme or implausible scenario."

At the Washington Post, Charles Krauthammer , who is a Fox News contributor, announced that "The Russia scandal has entered a new phase, and there's no going back." Stressing that he had been skeptical about the charges of possible collusion with the Russians, he writes : "The evidence is now shown. This is not hearsay, not fake news, not unsourced leaks. This is an email chain released by Donald Trump Jr. himself."

Now many people on the right would say, big deal. These are conservatives who write on the pages of the liberal mainstream media, so how much weight do they actually carry? The truth is, a lot. These are papers read by policymakers, and both men appear regularly on television to discuss their thoughts. But if this is not enough one can just turn to Fox News, the home of the conservative media, to see some cracks starting to emerge. Fox News has been extremely supportive of the Trump administration. As CNN's Brian Stelter has shown much of its coverage has echoed the talking points of the administration in response to each piece of bad news that emerges.

Last month, though, some frustration became evident when Neil Cavuto lashed out at the President for his continued attacks on news organizations such as CNN by saying : "Mr. President, it's not the 'fake-news media' that's your problem. It's you. It's not just your tweeting -- it's your scapegoating. It's your refusal to see that sometimes you're the one whose feeding your own beast -- and acting beastly with your own guys."

After the Donald Jr. emails were released, Chris Wallace said, "This really shouldn't be a matter of liberal v. conservative, pro-Trump vs. anti-Trump. If you're a fair-minded citizen, you ought to be concerned about the fact that we were repeatedly misled about what this meeting concerned."

Shepard Smith had what Aaron Blake of the Washington Post called his " Cronkite moment " when he told Wallace, "If there's nothing there -- and that's what they tell us, they tell us there's nothing to this and nothing came of it, there's a nothingburger, it wasn't even memorable, didn't write it down, didn't tell you about it, because it wasn't anything so I didn't even remember it -- with a Russian interpreter in the room at Trump Tower? If all of that, why these lies? Why is it lie after lie after lie? ... My grandmother used to say when first we practice to -- Oh what a tangled web we weave when first we practice to deceive. The deception, Chris, is mind-boggling."

One day we might look back at this moment as an important turning point in the Trump presidency. Historically, significant shifts among journalists in how they cover and analyze a story can have major political effects. The media has the power to sway public opinion.

In 1954, Edward Murrow broadcast a powerful episode of "See It Now" which exposed the contradictions and lies of rabid anti-Communist crusader Sen. Joseph McCarthy. "This is no time for men who oppose Sen. McCarthy's methods to keep silent," he said, "or for those who approve. We can deny our heritage and our history, but we cannot escape responsibility for the result ... The actions of the junior Senator from Wisconsin have caused alarm and dismay amongst our allies abroad, and given considerable comfort to our enemies. And whose fault is that? Not really his. He didn't create this situation of fear; he merely exploited it -- and rather successfully. Cassius was right. 'The fault, dear Brutus, is not in our stars, but in ourselves.'" Murrow's broadcast was an important moment in Sen. McCarthy's downfall.

Blake's reference to a "Cronkite moment" harked back to 1968 when CBS nightly news anchor Walter Cronkite, perhaps the most trusted voice in the media, ended his special broadcast about the Vietnam War's Tet Offensive by saying that, "To say that we are closer to victory today is to believe, in the face of the evidence, the optimists who have been wrong in the past ... To say that we are mired in stalemate seems the only realistic, yet unsatisfactory, conclusion."

Watching on one of his television sets in the Oval Office, Johnson told his aides, "If I've lost Cronkite, I've lost Middle America." Cronkite was not alone. Historians have documented how the increasingly critical coverage in the late 1960s fueled anti-war sentiment.

Follow CNN Opinion Join us on Twitter and Facebook

Of course, today's media landscape is different. The conservative media that has taken form since the 1970s has an especially strong hold on the beliefs of Republican voters. This has been very important to the political insulation that President Trump has enjoyed.

Unlike President Richard Nixon, he has been able to count on a well-developed and sophisticated conservative media universe. And, to be sure, some of the conservative media has tried to find creative ways of defending Trump Jr. and minimizing the revelations, even suggesting without any evidence that the approach to Donald Trump's son was part of a Democratic plot to entrap him.

It is much too early to tell if the voices of protest heard this week will turn into something bigger and more sustained, or if the majority of the coverage on these outlets remains pro-Trump. As Joshua Green wrote in The New York Times, most of the conservative media still clings to an "alternative reality" that fits President Trump's own narrative.