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.

This week's decision to fire FBI Director James Comey has greatly intensified the fears that our commander in chief sees absolutely no boundaries to his power.

By axing the director of an agency investigating whether there was collusion between members of his campaign and Russians, Trump risked being seen as obstructing a criminal investigation. He then piled on by tweeting out a threat to Comey should he leak information to the media. If Trump obstructed justice, it would be fair to say this scandal could be worse than Watergate.

In retrospect, the most revealing moment in the 2016 campaign occurred when Trump said at a campaign rally in January, "I could stand in the middle of 5th Avenue and shoot somebody and I wouldn't lose voters." The statement perfectly captures the mentality of the leader who holds the keys to power in the United States.

Besides his own sense of self, Trump's arrogance about what is possible comes from his counting on many aspects of our political system to protect him from political or legal punishment. There are some important differences between American politics in 2017 and 1974 that could insulate him from the fallout that President Richard Nixon faced.

Partisanship much stronger

Trump has always depended on the fact that partisanship is the most powerful force in the electorate. When he ran against Hillary Clinton, he anticipated that, in the end, Republicans would back him over any Democrat regardless of what he did or said. His dependence on partisanship remains a pillar of his political strategy. And there is reason for him to feel this way. It was notable that hours after the news of Comey's departure, Senate Majority Leader Mitch McConnell announced that he would not support the appointment of a special prosecutor.

The level of partisanship is much stronger than it was in final days of Nixon's presidency, when a group of Republicans, including Arizona Senator Barry Goldwater and House Minority Leader John Rhodes, finally approached the President to say that the time had come to step down. They told him he had lost his support in Congress.

Today there are real questions about whether Republican leaders would ever do the same thing. If most party leaders in the House and Senate hold firm in defense of the President, Trump understands that -- at least until the 2018 midterm elections -- party discipline would curtail the scale and scope of any congressional investigation.

Friends in the media

The news media is also very different than it was in the early 1970s. President Trump counts on the existence of powerful conservative news outlets that have the capacity and the will to counteract damaging news stories about what he has done.

In recent days, Fox News has been working hard to blame the media , saying it is blowing this story out of proportion. Their web site is also running a headline that the new FBI Chief could reopen the case about Hillary Clinton's emails, throwing their readers some red meat as the President struggles.

While conservative news shows feed the electorate with alternative narratives about what is going wrong in Washington, social media now floods voters with fake news that confuses many people about what they should believe. With so much disinformation circulating in the political ecosystem, President Trump will count on his opponents having trouble building sustained support for an investigation, and possibly impeachment.

Trump knows he can find support in the conservative media to counteract the 2017 version of Bob Woodward and Carl Bernstein, the reporters whose work helped lead to Nixon's resignation.

Changing the subject

Trump no doubt depends on the idea that the short attention span of the country helps him survive this crisis. We move at a much faster pace today than in the early 1970s.

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In this respect, the issue is not whether the public supports or opposes what he has done, but that Americans will quickly move on to the next story. President Trump will do everything in his power to make that happen with more outrageous statements and actions.

He can also accelerate discussions about high-profile policies such as tax cuts to redirect the energy of legislators who might otherwise focus on the Russia scandal. That tactic worked well in the campaign, during which voters could barely keep up with every new Trumpism. Effective investigations take time in our political system. The legislators who conduct them need to feel support from constituents over months, not days. It is not clear if the public can sustain this. If they don't, legislators will lose their sense of urgency about staying focused on this investigation.

During the campaign, Trump counted on the fact that many people hated politics enough that they would tolerate his outlandish ways. The President continues to rely on people concluding that today's news doesn't seem any worse than what they normally perceive to be going on in the "swamp" of Washington. Disillusionment with politics is so strong that public attitudes could insulate him.

But maybe, just maybe, this time President Trump went too far. The decision to fire James Comey and the string of lies that followed that decision feel different to many observers. President Trump seems to have really slipped into the realm of President Richard Nixon.

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There is ample evidence to suggest that the President either lashed out at Comey because he was angry about the Russia investigation or, even worse, is attempting to cover this up and obstruct the investigation. When Trump threatened Comey in a tweet, it was impossible not to feel like this was 1974 all over again. With stories emerging that he had asked Comey several times whether he would be loyal, the determination of President Trump to contain this investigation becomes clearer by the hour.

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The big question is not only what Trump did and when he did it, but whether the many parts of our political system that this president has counted on to save him will finally start to crumble. Most important is the question of whether there will be any profiles of courage -- Republican leaders who step in, just as they did in 1974, to say that this is enough and guarantee a full investigation as well as punitive actions should they be necessary. They should keep in mind that at this rate, when this story comes to an end, many historians might conclude that the comparison of the current situation to Watergate was unfair -- but to President Nixon rather than to Trump.

Americans can't assume that the system will work. It will take immense political pressure on legislators in the coming weeks to make certain that our leaders use every mechanism available to discover and prevent abuse of power and to ensure the health of our democracy for future generations.