This is a dangerous moment, which requires Congress and members of this administration to look beyond partisan maneuvering and tend to the health of the democracy itself.

In four tweets, capped by one about Arnold Schwarzenegger’s “pathetic” ratings on Celebrity Apprentice, Mr. Trump declared as fact a theory he apparently encountered on alt-right websites: “How low has President Obama gone to tapp [sic] my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!”

Mr. Obama issued a statement saying that neither he “nor any White House official ever ordered surveillance on any U.S. citizen.” James Clapper, the former director of national intelligence, denied on Sunday that the government had wiretapped Trump Tower before the election, and said he had no knowledge of any effort to do so before Mr. Obama left office.

The background for Mr. Trump’s outburst is, of course, the F.B.I.’s investigation of his inner circle’s contacts with Russian intelligence. It would be highly unusual for a president to be privy to details of a law enforcement investigation targeting his associates, let alone targeting him. If the inquiry is primarily a counterespionage investigation, however, he might properly have been briefed on it. Not much is known about this inquiry. The mere fact that a new administration is being investigated for potentially colluding with Moscow is uncharted territory.

Mr. Trump is now trying to bootstrap his claims into a congressional investigation of the Obama administration. On Sunday Sean Spicer, his press secretary, issued a statement demanding that congressional intelligence committees, led by Republicans friendly toward Mr. Trump, “determine whether executive branch investigative powers were abused in 2016.” Representative Devin Nunes, chairman of the House Permanent Select Committee on Intelligence and a member of Mr. Trump’s transition team, quickly made clear he intended to do the president’s bidding.