In dramatically casting aside James B. Comey, President Trump fired the man who may have helped make him president — and the man who potentially most threatened the future of his presidency.

Not since Watergate has a president dismissed the person leading an investigation bearing on him, and Mr. Trump’s decision late Tuesday afternoon drew instant comparisons to the “Saturday Night Massacre,” when President Richard M. Nixon ordered the firing of Archibald Cox, the special prosecutor looking into the so-called third-rate burglary that would eventually bring Nixon down.

In his letter informing Mr. Comey that he was terminated as F.B.I. director, Mr. Trump made a point of noting that Mr. Comey had three times told the president that he was not under investigation. But Mr. Comey has said publicly that the bureau is investigating Russia’s meddling in last year’s presidential election and whether any associates of Mr. Trump’s campaign were coordinating with Moscow.