Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book "Fault Lines: A History of the United States Since 1974." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) As we wait for Robert Mueller's report to drop and with House Democrats launching investigations into the administration, many are wondering if there will be a "smoking gun" that proves President Donald Trump deserves to be impeached.

Trying to find a smoking gun in a federal investigation is not new. In every investigation since Richard Nixon's Watergate scandal , politicians have assumed that it was necessary to discover a concrete piece of evidence that can prove with total certainty that a president committed "high crimes and misdemeanors." With Ronald Reagan's Iran Contra scandal, a smoking gun was never found. With President Bill Clinton, it was Monica Lewinsky's blue dress.

But the expectation of finding one piece of conclusive evidence is misplaced and too often obscures the big picture.

Not every investigation will be like Watergate, where just as the House Judiciary Committee voted in late July 1974 on articles of impeachment, the Supreme Court voted unanimously that President Nixon could not invoke executive privilege to block the release of White House recordings.

One of these tapes was from June 23, 1972 -- six days after the break-in of the Democratic National Headquarters at the Watergate complex -- in which President Nixon could be heard saying that he wanted the CIA to stop the FBI investigation.

There it was: obstruction of justice.

"The tape was the last straw," former national security adviser and Secretary of State Henry Kissinger recalled in TIME magazine . "It provided the pretext for waverers to commit themselves to impeachment and for others to abandon Nixon. By now there had been too many shocks; everybody wanted to get it over with." On August 5, 1974 , the public heard the tapes.

Two days later , upon seeing the full transcript, New York Republican Rep. Barber Conable said it "looked like a smoking gun." Republican Sens. Barry Goldwater and Hugh Scott met with the President to tell him he was finished. On August 8, Nixon announced his resignation.

Ever since that epic moment in American history, the "smoking gun" has achieved a mythological status in presidential scandals. The problem is that the story exaggerates the role of this kind of evidence.

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Indeed, even with Nixon, the House Judiciary Committee had already voted in favor of three articles of impeachment, with Republican support, before the tape became public. The decision came after months of congressional and prosecutorial investigations -- based on witness testimony and documentary evidence -- that had put together a picture of how Nixon had abused his presidential power. The smoking gun tape had its power as a "closer" only because of all the work Congress had already done.

Of course, it could be that there is a comparable smoking gun that will be found today, relating to contacts between Trump's team and Russian President Vladimir Putin.

But it is most likely that there is no smoking gun coming. Nothing of that nature has surfaced so far that we know of. And in his testimony before Congress, Michael Cohen made it clear, even if there was collusion, that President Trump is more careful than that. Within his organization, he allegedly sends his signals about what to do through code and mutual understandings. "He doesn't give you questions," Cohen told the Oversight Committee , "he doesn't give you orders, he speaks in code. And I understand the code, because I've been around him for a decade."

Congress should not focus on a smoking gun nor should the committees make this the heart of their inquiry. Even without one singular piece of evidence that "proves" everything, there might be enough from the data Congress does have, as well as testimony and corroborating evidence, to make a strong case for triggering the formal impeachment process.

Instead, congressional investigators need to answer a set of bigger questions to determine whether impeachment proceedings are necessary before the 2020 election begins. Some of their job will be to determine how seriously to take the ways in which the President has openly obstructed justice. Unlike President Nixon, Trump has done much of his dirty work in public. As House Judiciary Committee Chairman Jerry Nadler said last weekend , it is "very clear" the President has obstructed justice.

There is a strong case to be made just from the public record that Trump has "prevented, obstructed, and impeded the administration of justice," to quote the first article of impeachment against Nixon. The President has castigated Justice Department officials, spread doubts about the legitimacy of law enforcement and intelligence institutions, even accused former FBI Deputy Director Andrew McCabe of attempting a coup.

He fired FBI Director James Comey and then bragged to Russian officials that it had taken "great pressure" off the Russia investigation and told NBC News with the cameras rolling that the firing was because of the "Russia thing." He has also used Twitter to praise witnesses, such as Roger Stone, when they didn't cooperate with Mueller.

He fired Attorney General Jeff Sessions, who multiple reports have found was a target of his ire because he recused himself from the Russia investigation, and replaced him with acting Attorney General Matthew Whitaker and then Attorney General William Barr, both of whom have been critical of the Mueller investigation.

These actions come on top of a New York Times report that the President leaned on Whitaker to have US Attorney Geoffrey Berman, a Trump ally, un-recuse himself so he could oversee Mueller's investigation.

There has also been extensive reporting and public testimony that has suggested the problem is even deeper than what we have seen, such as Trump helping to draft a false account of the June 9, 2016, meeting at Trump Tower. A report from the Brookings Institution , co-written by Norm Eisen, who is now working for the Judiciary Committee, offered extensive documentation of the evidence Congress already has.

The congressional committees must also try to understand whether the President's personal political and economic interest guided public policy decisions. This gets to the heart of the conflict-of-interest problem that has always faced this administration.

With a businessman in the Oval Office who refused to totally separate himself and his family from the business, the government has been vulnerable to major conflict-of-interest problems at almost every turn. These conflicts are relevant to the Russia investigation because it turns out the President was negotiating a deal throughout the campaign to build a Trump Tower in Moscow. This issue is relevant to understanding what happened at the inauguration, when there was questionable spending by Trump's inaugural committee at the Trump International Hotel in Washington.

Finally, Congress must look into whether Trump is abusing his presidential power. In her blockbuster piece for the New Yorker, Jane Mayer put the problem front and center with her reporting that the President tried to block the merger of Time Warner and AT&T apparently because of his anger with CNN's coverage.

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His aggressive use of presidential power to secure funds for a border wall, based on a manufactured crisis at the border, has even caused some Republicans to finally break with the administration. The New York Times and CNN have published reports that he pressured his staff to give his daughter Ivanka and son-in-law Jared Kushner security clearances despite recommendations against doing so. Ever since Nixon, the abuse of presidential power has been at the heart of what drives Congress to take action against the White House.

So, rather than focus on a hunt for smoking guns, Congress needs to be helping the public sort through these three big issues and make sense, through questioning witnesses and finding documents, of what the Trump presidency is all about.

In the end, the committees might find that while the President took many unethical and even illegal actions, there is not sufficient political support to move forward with impeachment proceedings. Or, they might reach a different conclusion, finding that as a result of the evidence they have no option other than to formally consider impeachment if they want to fulfill their constitutional obligation to preserve our system of checks and balances.