Editor’s Note: 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.

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House Speaker Nancy Pelosi just announced that Congress is launching a formal impeachment inquiry. The allegations that President Donald Trump withheld military aid to Ukraine in an effort to persuade Volodymyr Zelensky to investigate Democratic candidate Joe Biden has turned out to be a genuine tipping point. Seven moderate Democrats with previous military service published an op-ed in the Washington Post. “We believe these actions represent an impeachable offense,” they said.

After several months, when Pelosi resisted taking this step for fear of a political backlash, President Trump finally found a way to go too far. It turns out that there might still be a line in American politics, and he just crossed it.

As Democrats officially start the impeachment process and undertake the investigation, they will continue to be haunted by historical comparisons.

Democrats who are leery about moving forward see the late-1990s impeachment of Bill Clinton all over again. With the 2020 election in the balance, they fear that their party will end up like the Republicans, moving forward to impeach the President only to encounter a fierce political backlash against their party for seeming too extreme.

Other Democrats, who have been calling for impeachment since the release of Robert Mueller’s report, see a repeat of the early-1970s. In their minds, Trump is the new Richard Nixon, just on steroids. For them, moving forward with impeachment is not only the right thing to do but it will also strengthen the Democrats politically, by putting them on the right side of history.

The truth is we don’t know how this will turn out. The trouble with speculation is that the future remains unpredictable. We can make educated guesses about how the politics might unfold and we can look to previous examples to better understand what is happening. But at the end of the day, the impeachment of Trump must be understood on its own terms.

This process will play out within the particular dynamics of 2019 and 2020. It is easy to map out how either prediction could be true. At the same time, it is even easier to imagine how we can’t really envision or anticipate what is going to happen and what the ultimate political effects will be.

The impeachment process is always especially uncertain since it revolves around an intense investigation that sheds light on a situation rather than confirming what we already know. This means that, if the process is handled well, we will have many more facts on the table before any vote for articles of impeachment takes place.

While we live in an era where facts often don’t matter and the people make decisions based on partisan preferences, dramatic revelations can still shake the status quo. That’s what happened with Nixon – the process itself dramatically changed public opinion. We have just seen that within the past few hours, as the heads of even the most seasoned Washington watchers are spinning.

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    Democrats need to make their decisions based on the obligations of the office and a respect for our democratic institutions. If there is one big lesson from the last two of our three impeachment processes – it is that voters rewarded the party that in 1974 took this bold step but did so in a serious way, focusing on the health of our democratic institutions and the balance of power. When the Republican party seemed to move forward in a haphazard and clearly partisan manner in 1997 and 1998, the public disapproved of their actions.

    Although politics is naturally always on the mind of any elected official, this is a moment when they need to make those considerations secondary. It looks like Democrats have finally reached a point in their thinking, where they are willing to do what is right rather than what might be the easiest political path.

    The outstanding question remains the Republicans and whether the revelations that keep emerging are enough to shake them out of a partisan slumber than has led almost all of them to be blindly loyal regardless of what the President does or says. In his latest statement, Senate Majority Leader Mitch McConnell did not instantly jump to the President’s side, vowing to “find out what happened.” Given his history, the odds are that McConnell will stand firmly behind the Republican President. But the thing you learn from history is that politics can move in unexpected directions at the most unexpected of times.