Julian Zelizer is a history and public affairs professor at Princeton University and the editor of " The Presidency of Barack Obama: A First Historical Assessment ." He's also the co-host of the "Politics & Polls" podcast. Follow him on Twitter: @julianzelizer . The opinions expressed in this commentary are his own.

(CNN) Former FBI Director James Comey has some tough words for President Donald Trump. In "A Higher Loyalty," Comey writes that the President is "unethical, and untethered to truth and institutional values. His leadership is transactional, ego driven and about personal loyalty."

He compares Trump to the mob bosses he encountered as a federal prosecutor. "The us-versus-them worldview. The lying about all things, large and small, in service to some code of loyalty that put the organization above morality and above the truth." He warns those who have tuned out that "What is happening now is not normal. It is not fake news. It is not okay."

Now comes the tough part. To fight back against Comey, Trump's allies have launched a campaign to discredit him as a self-serving, disingenuous, attention-seeking ex-lawman who just wants to sell books. "Lyin' Comey," the Republican National Committee is calling him, on a bizarre website that includes derogatory quotes from Hillary Clinton, Sen. Charles Schumer and Rep. Maxine Waters slamming Comey about his role in the 2016 election.

If you think that such a campaign to discredit Trump opponents can't work, think again. After months of sustained attacks, a new CNN poll says 54% of Republicans now believe that Robert Mueller is not conducting a fair investigation into the administration (this is up 10 percentage points from six months ago). If the numbers keep going up, the odds of Republicans doing anything about a scandalous Mueller report keep going down.

Kellyanne Conway, the President's favorite go-to television spokeswoman in times of crisis, said that Comey has a "revisionist view of history and seems like a disgruntled ex-employee." The President, she said, "has a different recollection of what happened."

JUST WATCHED White House: Comey a 'disgraced partisan hack' Replay More Videos ... MUST WATCH White House: Comey a 'disgraced partisan hack' 00:52

Not to be outdone, President Trump let off a tweet-rant that Comey is a "proven LEAKER & LIAR" and that "Virtually everyone in Washington thought he should be fired for the terrible job he did -- until he was, in fact, fired." Apparently insisting on preserving some level of presidential decorum, Trump went on to say that "He is a weak and untruthful slime ball who was, as time has proven, a terrible Director of the FBI."

In the coming weeks, starting with his interview on a special "20/20" with George Stephanopoulos, who knows something about presidential scandals, the question will be whether Comey's appearances can overcome the onslaught of attacks.

Here are some of the things to watch for as Comey's book tour unfolds:

Did President Trump obstruct justice?

More than anything else, the question of obstruction is the one place where Comey can be extremely dangerous to the President. Comey's story has been a pivotal piece in the debate about whether Trump attempted to stifle the FBI investigation about Michael Flynn and other concerns about Russia. This was the issue that emerged through the fears Comey expressed in his memos and his testimony to Congress.

Obstruction charges can be extremely damaging politically, as Presidents Richard Nixon and Bill Clinton each learned, as they hold potential to fuel impeachment proceedings, should Democrats gain control of Congress this year. If the book rollout provides more credible evidence about the President having obstructed justice, then Trump should be concerned.

Does Comey's book further erode the image of President Trump?

The damage that Comey's appearances around the book might inflict on Trump's already battered reputation might be more significant than anything else he could do. After all, the fate of the President will ultimately be decided in the political arena. The greater the damage to the President's political standing in Congress and within the electorate, the greater the odds Republicans will suffer in the midterm elections, that another Republican will challenge Trump in the primaries, and that Democrats will have a better chance in 2020.

Can Comey avoid looking like he's just out to sell books?

From the moment he re-enters the public stage, Comey runs the risk of playing directly into President Trump's hands. Of course, Comey wants to make sure that his book gets read. He will participate in the traditional high-profile appearances that should be expected with someone of his stature.

JUST WATCHED Sanders: We shouldn't be praising Comey Replay More Videos ... MUST WATCH Sanders: We shouldn't be praising Comey 01:12

But this book tour is different than many others. The political stakes are immense. The President will try to make Comey's interest in selling books a reason not to believe what he has actually written. This is the catch-22 that the former FBI director confronts. Each appearance will give President Trump the opportunity to get onto Twitter and send out a nasty-gram about Comey's real intentions. Comey's demeanor, his choice of words, and his temperament will be very important to rebutting Trump allies who say: Don't listen.

Winning over Democrats?

Although all the attention is on Comey and Trump, Democrats have as many questions as does the GOP. Comey has to convince them that he is a person they can trust. Given all the slander coming from the President and his allies, it is easy to forget that Hillary Clinton was actually the candidate harmed most by Comey in the election, not Trump. As the RNC website reminds its readers, it was Hillary Clinton -- not Donald Trump -- who paid the steepest price for Comey's controversial announcement about the email investigation continuing in late October.

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Many Democrats want to hear more about how and why he made the decision. In his book, Comey says he wanted to make sure that Clinton would not appear illegitimate if she was the victor. This might not cut it with many Democrats still in disbelief about his decision. Until he makes a convincing case about it, a large number of Democrats will also continue to look at him with skepticism.

Can Comey shock and awe?

In an era where every story seems to come out in real time, this is a big question facing Comey: In his interviews, with the book, and in every one of his appearances, is there anything that he can do which will shock us? Or are we comfortably numb? Maybe the real shock is that the pundits keep predicting that there will be one. Comparing the President to mob boss, for example, doesn't seem so extreme anymore. Warning that the President doesn't care about the truth or is destroying the institution of the presidency are already part of our lived reality. It could be that Comey discovers the American people so swamped with scandalous behavior from the Oval Office that nothing really moves them.

This will be one of the greatest challenges as he talks about the book: getting the shell-shocked electorate to pay attention, absorb what he has to say and consider that his experiences really point to something that can't be dismissed as "normal."