Donald Trump should reveal what he knows about an alleged wire tap of his phones ordered by Barack Obama before November's election, says a Republican senator.
Ben Sasse called the US President's surveillance claims serious and said the public deserved more information.
The politician said it was possible the tycoon had been illegally tapped at Trump Tower, but, if so, he should explain what sort of device it was and how he knew about it.
Mr Trump has not offered any evidence to support his claims, and Mr Obama has rejected the allegations, saying they were "simply false".
On Saturday Mr Trump tweeted: "Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
The President added: "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!"
"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!"
And in an additional message, he said: "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!"
But Kevin Lewis, a spokesman for Mr Obama, rejected the allegations, saying: "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 added: "As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any US citizen.
"Any suggestion otherwise is simply false."
House Democratic Leader Nancy Pelosi ridiculed President Trump, writing: "The Deflector-in-Chief is at it again. An investigation by an independent commission is the only answer."
McCarthyism is defined as making allegations of subversion or treason with insufficient regard for evidence.
It is said to have begun with an executive order issued by President Harry Truman in March 1947, requiring all federal employees within the American civil service to be screened for "loyalty".
Senator Joseph McCarthy claimed in 1950 that known communists were working in the US State Department.
Later, in 1954, he was censured by the Senate.
The Watergate political scandal followed a break in at the Democratic National Committee's HQ in the Watergate office building in Washington DC in 1972.
Multiple abuses of power by President Nixon's administration were subsequently discovered, eventually forcing his resignation.