Donald Trump has accused Barack Obama of tapping his phone during the US election.
The president tweeted that his predecessor had his "wires tapped in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
He also says a good lawyer could "make a great case" out of the allegation.
Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A NEW LOW!— Donald J. Trump (@realDonaldTrump) March 4, 2017
The president compared the alleged surveillance of his communications to Watergate, the scandal in the early 1970s that brought down Republican president Richard Nixon.
He offered no evidence or said what prompted the allegation.
Mr Obama's office is yet to respond.
I'd bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to Election!— Donald J. Trump (@realDonaldTrump) March 4, 2017
It is thought Mr Trump's tweets could be in response to Democrats' outcry following revelations that Attorney General Jeff Sessions - in his confirmation hearings - did not disclose his contacts with Russia's American ambassador during the campaign.
Mr Sessions, who was a senator at that time, was Mr Trump's earliest Senate supporter.
US intelligence agencies have concluded Russia influenced the election to help Mr Trump win.
How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!— Donald J. Trump (@realDonaldTrump) March 4, 2017