Julian Zelizer , a history and public affairs professor at Princeton University and a New America fellow, 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) President Trump dropped a bombshell Tuesday with the announcement that he had fired FBI Director James Comey -- just days after Comey testified before the Senate Judiciary Committee about, among other things, the bureau's investigation into Russian meddling in the election that propelled Trump to the presidency.

Trump has stunned the political world once again by issuing the orders to remove one of the most important figures in this entire investigation.

Ironically, the announcement comes at a time when former secretary of state and presidential candidate Hillary Clinton has been telling audiences that Comey was a key factor behind her loss in November. His infamous announcement in late October that the FBI was looking into new emails revived the specter of the earlier probe into Clinton's emails just before voters went to the polls. Many experts agree that the announcement cost her points with voters.

But then Comey turned into a problem for President Trump. The Russia investigation has hung over Trump like a dark cloud since his first days in office. Even as congressional committees have stumbled over partisanship in their own probes of Russia's interference, the FBI seems to have been driving forward at an aggressive pace, continuing to give strong indications about evidence that Trump campaign officials were in contact with Russian officials during the campaign.

While Trump has continually denied any collusion and has lobbed accusations of his own, the FBI appears to have been keeping its eye on the ball.

President Trump's decision to fire Comey is the second such dismissal to rock the administration. When the President announced the resignation of Acting Attorney General Sally Yates in January for refusing to implement the administration's refugee ban, the comparison to President Richard Nixon's "Saturday Night Massacre" instantly lit up the headlines.

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In that 1973 event, Nixon fired independent prosecutor Archibald Cox for his aggressive inquiry into Watergate. It turns out that was nothing, compared with the firing of Comey in the middle of this inquiry. It is a stunning blow to any attempt to obtain legitimate, non-partisan information about what went wrong in the campaign and why.

The justification provided in the administration's memorandum on the firing -- that it came in response to Comey's mishandling of the Clinton investigation -- doesn't pass the laugh test.

While it is true that candidate Trump has complained that Comey gave Clinton "a free pass" in his decision not to bring criminal charges against her, Trump and his surrogates capitalized on that investigation more than anyone else. The President refused to take any action on that front until now. This is not about Hillary Clinton.

There is no reason that the public should trust the congressional committees to do the job any longer. Indeed the House investigation completely broke down when it became clear that the chair of the committee, Congressman Devin Nunes, displayed more loyalty to the party and President than to finding out the truth.

The Senate committee then stalled and delayed until criticism finally pushed it toward doing its job. But the power of partisanship remains strong, and many observers are skeptical that Senate Republicans, like Texas' Ted Cruz, are willing to go where the facts take them.

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From the start, President Trump has showed little interest in finding out what happened during the election. This has been one of the most suspicious aspects of his response. He has attacked President Barack Obama with false allegations of wiretapping, he has dismissed all the evidence and accusations coming out about campaign officials, and he has taken a strident stand against all the institutions that show any sign of standing up to him.

The reason that the Saturday Night Massacre stung President Nixon so badly was because the firing proved to the public that the President really was frightened that the truth would come out. He was unwilling to let the institutions of government do their job, and the decision to get rid of Archibald Cox demonstrated that the President would do anything to protect his interests.

This part of the Watergate cover-up, and, even worse, the effort to actively fight the investigation, turned public opinion against him.

The question now is whether Trump has enough Teflon support from his base to keep even this from hurting his standing. His public approval ratings are low; it's unclear how much lower they can sink. But there are several other ways in which he is vulnerable.

Firing Comey could begin to make a dent in the strong support that he has enjoyed among Republicans, who will recognize this as a blatant effort to circumvent the law and cover up the truth. The firing will also put Congress -- both parties -- on notice that this is a President who will do almost anything to protect himself. Today's firing could encourage the leaders of both parties to double down with their investigations.

More than anything else thus far, this announcement fuels the perception that President Trump is scared about something. There was no obvious reason for the administration to take this step and it's hard not to be skeptical about the reasons for it to be taken at this moment.

From the day he stepped into the White House, President Trump has raised concerns that he does not adequately respect the boundaries of power. He has dismissed concerns about the conflicts of interest with his family business, he has openly attacked judges and Congress as illegitimate, and he has attempted to use executive power in an aggressive fashion.

Firing James Comey right in the middle of the Russia debate looks like President Trump's nuclear option to dealing with an investigation into the very foundations of his power.

The Senate has a big job to do, and Senate Republicans will need to insist, through the power of confirmation, that President Trump appoint someone of the highest standing to replace James Comey and to see that this investigation is allowed to go wherever it might take the agency.

Senate Republicans must show that they are more loyal to the nation than their party. Democrats, who have little love for Comey after his pronouncements about Clinton before the election, need to show that they can work with the GOP on this issue to make sure someone strong takes over the job. Better yet would be to take up Senator Chuck Schumer's plan to appoint a special prosecutor, which is now the only way to move forward with a serious investigation.

Without such action, the legitimacy of the 2016 election and the legitimacy of this President will remain a question. Congress must rectifying this imperial act -- or the health of our democracy will continue to hang in the balance.