US President Donald Trump has been asked to provide evidence to back his allegation that his predecessor, Barack Obama, ordered his phones to be tapped during the election campaign.
Republican Senator Ben Sasse said Mr Trump’s comments were “serious” and he should explain the alleged wire-tapping and how he came to know about it.
The Republican president has supplied no details to back his words.
An Obama spokesman said he had never ordered surveillance of any US citizen.
Under US law, wire-tapping can only be approved if there is probable cause to believe that the target is an agent of a foreign power.
Mr Trump, who has been facing intense scrutiny over alleged Russian interference in support of his election campaign, made the allegation on Saturday.
He tweeted: “Terrible! Just found out Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!”
The spokesman for Mr Obama, Kevin Lewis, said the accusation was “simply false”.
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.
The statement left open the possibility that a judicial investigation had been taking place.
Earlier Ben Rhodes, who was Mr Obama’s foreign policy adviser and speechwriter, also addressed Mr Trump’s claims in a tweet, saying: “No President can order a wire-tap. Those restrictions were put in place to protect citizens from people like you.”
Mr Trump, who is at his Florida resort, fired off a series of tweets from just after 06:30 local time (11:30 GMT) on Saturday.
He called the alleged tapping “a new low” and said “This is Nixon/Watergate” – referring to the most notorious political scandal of 1972, which led to the downfall of President Richard Nixon after a web of political spying, sabotage and bribery was exposed by the media.
McCarthyism, which Mr Trump referred to in one of the first posts, relates to the persecution of US Communists and their allies led by Senator Joe McCarthy in the 1950s.