President Trump on Saturday accused former President Barack Obama of tapping his phones at Trump Tower the month before the election, taking to Twitter to call his predecessor a “bad (or sick) guy.”

Without offering any evidence or Mr. Trump fired off a series of Twitter messages claiming that Mr. Obama “had my ‘wires tapped.’ ” He likened the supposed tapping to “Watergate/Nixon” and “McCarthyism.”
Mr. Trump’s aides declined to clarify whether the president’s explosive allegations were based on briefings from intelligence or law enforcement officials or on something else, like a news report.
A spokesman for Mr. Obama, Kevin Lewis, issued a statement dismissing the claims. “A cardinal rule of the Obama administration was that no White House official ever interfered with any independent investigation led by the Department of Justice,” he said. “As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen. Any suggestion otherwise is simply false.”
Officials from the Obama administration called Mr. Trump’s accusation shocking and untrue. They pointed to longstanding laws and procedures intended to ensure that presidents cannot wiretap a rival for political purposes.
Mr. Trump’s decision to lend the power of his office to such a claim — without offering any proof — was remarkable, even for a leader who has repeatedly shown himself willing to make assertions that are false or based on rumors.
It would have been difficult for federal agents, working within the law, to obtain a wiretap order to target Mr. Trump’s phone conversations. It would have meant that the Justice Department had gathered sufficient evidence to persuade a federal judge that there was probable cause to believe he had committed a serious crime or was an agent of a foreign power, depending on whether it was a criminal investigation or a foreign intelligence one.
In one message, which Mr. Trump sent from his Palm Beach, Fla., resort at 6:35 a.m., the president said he had “just found out” that his phones had been tapped before the election. Speculation online quickly turned to the possibility that Mr. Trump had been reading an article on the Breitbart News site or listening to the conservative radio host Mark Levin; both have embraced the theory in recent days.
The Breitbart article, published on Friday, claimed that there was a series of “known steps taken by President Barack Obama’s administration in its last months to undermine Donald Trump’s presidential campaign and, later, his new administration.”
One former senior law enforcement official who worked under Mr. Obama said that it was “100 percent untrue” that the government had wiretapped Mr. Trump, and that the current president should be pressed to offer any evidence for his assertion.
Ben Rhodes, a former top national security aide to Mr. Obama, said in a Twitter message directed at Mr. Trump on Saturday that “no president can order a wiretap” and added, “Those restrictions were put in place to protect citizens from people like you.”
It has been widely reported that there is a federal investigation, which began during the 2016 presidential campaign, into links between Trump associates and the Russians.
The New York Times reported in January that among the associates whose links to Russia are being scrutinized are Paul Manafort, Mr. Trump’s onetime campaign chairman; Carter Page, a businessman and foreign policy adviser to the campaign; and Roger Stone, a longtime Republican operative who has said he was in touch with WikiLeaks before it released a trove of Democratic National Committee emails last summer.
Mr. Trump appeared on Saturday to suggest that warrants had been issued by the Foreign Intelligence Surveillance Court, claiming that Mr. Obama’s administration had once been “turned down by court” in its supposed efforts to listen in on conversations by Mr. Trump and his associates.
In the fall, the F.B.I. examined computer data showing an odd stream of activity between a Trump Organization server and Alfa Bank, one of Russia’s biggest banks, whose owners have longstanding ties to President Vladimir V. Putin. The F.B.I. concluded at the time that there may have been a benign explanation for the communications, like marketing email or spam. It is not clear whether the bureau obtained a warrant from the Foreign Intelligence Surveillance Court, known as a FISA warrant, as part of that line of inquiry; there is no evidence it did.
During the transition, the F.B.I. — which uses FISA warrants to eavesdrop on the communications of foreign leaders inside the United States — overheard conversations between the Russian ambassador to the United States and Michael T. Flynn, whom Mr. Trump had named national security adviser.
Mr. Trump has pointedly and repeatedly questioned in conversations how it was that Mr. Flynn’s conversations were recorded, and wondered who could have issued a warrant.
After The Washington Post reported that Mr. Flynn and the ambassador, Sergey I. Kislyak, had discussed sanctions that the Obama administration had just imposed on Russia, Mr. Flynn was pushed out of his post by the White House because he had lied to Vice President Mike Pence about the nature of the calls.
Mr. Trump’s allegations represented a sharp change in his tone toward Mr. Obama. The current president has frequently spoken about how much he admires Mr. Obama for the gracious way he handled the transition. But since taking office, Mr. Trump has frequently clashed with the intelligence agencies over the Russia inquiries, including efforts to examine the attempts by that country to influence the presidential election and the contacts between Mr. Trump’s aides and the Russian government.
In recent days, the president has appeared increasingly angry about leaks of information that he believes are coming from law enforcement and intelligence officials who are holdovers or recently departed from Mr. Obama’s administration.
People close to Mr. Trump have described him as determined to stop those people from sabotaging his administration. Across the government, efforts are underway to find and punish officials who are leaking information to the news media.
Mr. Trump’s mood was said to be explosive before he departed for his weekend in Florida, with an episode in which he vented at his staff. The president’s ire was trained in particular on Donald F. McGahn, his White House counsel, according to two people briefed on the matter.
Mr. Trump was said to be frustrated about the decision by Jeff Sessions, his attorney general, to recuse himself from participating in any investigations of connections between the Trump campaign and Russia. Mr. Trump has said there were no such connections.