Depending on what is eventually proven, the core scandal could rival Watergate, in which a “third-rate burglary” of Democratic National Committee headquarters ultimately revealed a wide-ranging campaign of political sabotage and spying to influence the 1972 presidential election and undercut perceived rivals. In the current case, a hostile foreign power sought to sway the 2016 election and there is evidence that at least some people in Mr. Trump’s circle were willing to collaborate with it to do so.

Mr. Nixon ordered the firing of Archibald Cox, the Watergate special prosecutor, and was ultimately undone by a secret tape recording of him requesting that his aides use the C.I.A. to impede the F.B.I. investigation into the burglary. Mr. Trump fired James B. Comey, the F.B.I. director originally leading the investigation, and has repeatedly pressured the Justice Department and special counsel to shut down the inquiry and investigate his Democratic rivals instead.

“Watergate pales, really, in my view, compared to what we’re confronting now,” James R. Clapper Jr., the former director of national intelligence, has said. David Corn, an author of “Russian Roulette,” a book about the case, has described it as “much bigger than what happened in Watergate.” After Mr. Comey’s dismissal, James Fallows wrote in The Atlantic that “this scandal looks worse than Watergate.” Carl Bernstein, one of the Washington Post journalists who led the coverage of the Nixon scandal, has said this appears “worse than Watergate in many, many ways.”

Like Hillary Clinton and the “vast right wing conspiracy,” Mr. Trump and his allies are trying to focus attention on the conduct of their pursuers, arguing that the F.B.I. or the special counsel investigating Russia’s involvement in the election have themselves stepped over the line. The president’s tweet on Thursday seized on a column in a conservative magazine asserting that the F.B.I. most likely had a confidential informant inside Mr. Trump’s campaign in 2016.

Mr. Trump extrapolated that to imply that his predecessor, President Barack Obama, had planted a spy on his team. “Wow, word seems to be coming out that the Obama FBI ‘SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT,’” he wrote. “If so,” he added, “this is bigger than Watergate!”