WASHINGTON, D.C.—Journalism icon Bob Woodward sees the tumult of President Donald Trump’s first year in office and understands that people—on both sides of the political aisle—think they know how the “final exam of American democracy” will unfold. But Mr. Woodward, the ACEP17 keynote speaker Sunday, cautioned against judging too quickly, as he once did.

The Washington Post associate editor, who helped report and investigate the Watergate scandal that pushed President Richard Nixon to resign in 1974, thought he knew the story of President Gerald Ford pardoning Nixon shortly thereafter.

“It’s the final corruption of Watergate,” Mr. Woodward told a crowded ballroom of emergency physicians. “There was an aroma of a deal between Nixon and Ford. Fordgets the presidency. Nixon resigns, but is pardoned.”

But some 25 years later, Mr. Woodward spoke to Ford for the first time about why he pardoned Nixon. “Ford said, in this plaintive voice I will never forget, ‘You know, I needed my own presidency,’” Mr. Woodward said. “‘The country had to move on. I had to move on. The only way to get Nixon off the front page and into history was to pardon him. That was the national interest.’”

The pardon helped cost Ford the 1976 presidential election, but the lesson to Mr. Woodward was that “the lens of history” frames a different picture. It’s a valuable perspective as President Trump’s “energized” pace prompts the media to forecast the future and question his motives.

“I was so sure I knew in ‘74, not just what had happened, but what it meant,” Mr. Woodward said, adding, “It is a lesson that will never leave me and it is a lesson that we sit in this moment in history, which is a really, really important moment, and I think the stakes could not be higher about what Trump’s going to do, but we don’t know.”

Mr. Woodward said the media must be careful in its tone and coverage of President Trump, lest its credibility be further eroded. And it has to “entertain the possibility … that Trump is operating in good faith” to do the job of the president.

So how does Mr. Woodward, who has written books on eight presidents and is working on his ninth, define the job description for the world’s most powerful man?

“To figure out what the next stage of good is for a majority of people in the country,” he said, “A real majority. Not one party, not interest groups, not a series of interest groups. But really step back and say, ‘What do we need?’”