Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the forthcoming book, "Burning Down the House: Newt Gingrich, the Fall of a Speaker, and the Rise of the New Republican Party." Follow him on Twitter: @julianzelizer. The views expressed in this commentary are his own. View more opinion at CNN.

(CNN) The Justice Department is reportedly conducting a criminal investigation into its own probe of Russian interference in the 2016 election — a decision that seems to be a brazen effort to politicize the department.

Democratic Reps. Jerry Nadler and Adam Schiff released a press statement expressing their concern about how the department had "lost its independence and become a vehicle for President Trump's political revenge."

Julian Zelizer

But the news should not come as a surprise to anyone who has been paying attention. The investigation is just a small taste of the kind of campaign the nation will experience in the run up to the 2020 election. The combination of Trumpian politics — character assassination, smears, conspiracy theories, and smashmouth attacks on opponents — combined with his use of presidential power will be unlike anything we have seen before. This will be a whole new kind of ugly.

While some political experts seem to be making horse race prognostications based on the assumption that this election will be business as usual, we should expect a campaign that is far nastier than the norm. We should expect all the remaining guardrails to fall away. Assuming President Trump survives the impeachment process, he will undoubtedly unleash a fierce assault on whoever his opponent ends up being. To overcome his weak standing in the polls , the President will try to tear everyone and everything else down so that he is the only one left standing.

What might the campaign look like? Official investigations into anything tied to the Democratic Party that Trump can find a way to publicly justify will certainly be a key part of the strategy. As the new DOJ criminal investigation reveals, the president is not above using the formal apparatus of government to conduct probes that aim to delegitimize his opponents.

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