WASHINGTON — As a candidate, Donald J. Trump claimed that the United States government had known in advance about the Sept. 11 attacks. He hinted that Antonin Scalia, a Supreme Court justice who died in his sleep two years ago, had been murdered. And for years, Mr. Trump pushed the notion that President Barack Obama had been born in Kenya rather than Honolulu, making him ineligible for the presidency.

None of that was true.

Last week, President Trump promoted new, unconfirmed accusations to suit his political narrative: that a “criminal deep state” element within Mr. Obama’s government planted a spy deep inside his presidential campaign to help his rival, Hillary Clinton, win — a scheme he branded “Spygate.” It was the latest indication that a president who has for decades trafficked in conspiracy theories has brought them from the fringes of public discourse to the Oval Office.

Now that he is president, Mr. Trump’s baseless stories of secret plots by powerful interests appear to be having a distinct effect. Among critics, they have fanned fears that he is eroding public trust in institutions, undermining the idea of objective truth and sowing widespread suspicions about the government and news media that mirror his own.

“The effect on the life of the nation of a president inventing conspiracy theories in order to distract attention from legitimate investigations or other things he dislikes is corrosive,” said Jon Meacham, a presidential historian and biographer. “The diabolical brilliance of the Trump strategy of disinformation is that many people are simply going to hear the charges and countercharges, and decide that there must be something to them because the president of the United States is saying them.”