US President Donald Trump has been urged 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!"
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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.
In other developments:
- ABC News quoted senior White House sources as saying President Trump had gone "ballistic" at an Oval Office meeting on Friday, in particular at the decision of Attorney General Jeff Sessions to remove himself from an investigation into alleged Russian interference in the election
- Politico and CNN report that Mr Trump will sign as early as Monday a new executive order imposing a travel ban on people from some Muslim-majority nations. The first was halted in the courts
- Trump supporters have held scores of rallies in locations across the country, ranging from several dozen people to the low hundreds, under the banner of the Spirit of America
Partisan maelstrom: Analysis by Anthony Zurcher, BBC News, WashingtonImage copyright Reuters
As the Trump administration continues to be bedevilled by a drip-drip of revelations about ties to Russian officials during and after last year's election, the president has seemingly settled on the identity of the malevolent figure behind the turmoil.
It was the former president, Mr Trump asserts, who is guilty of meddling in the 2016 campaign, not Russia. Mr Obama, he says, is the one whose deeds merit investigation.
The president's early morning tweets follow an interview on Tuesday in which he accused Mr Obama and "his people" of orchestrating recent political protests across the US and of being behind the government leaks that have embarrassed the White House.
There is scant evidence supporting these allegations, but charges like these fit a pattern. Mr Trump is at his sharpest when pushing back against perceived antagonists, such as Republican primary opponents like Jeb Bush, establishment conservatives who resisted his nomination or Hillary Clinton in the election.
Now Mr Trump is returning to his favourite political foil - a necessity given the current Democratic power vacuum in Washington. It could mark the beginning of a massive water-muddying effort in which any forthcoming investigatory revelations are swept up in a growing partisan maelstrom.
Mr Trump's tweets followed allegations made by conservative radio host Mark Levin.
Mr Levin said there should be a congressional investigation into what he called Mr Obama's "police state" tactics in his last months in office to undermine Mr Trump's campaign.
Some Democrats have suggested Mr Trump's latest tweets are an attempt to focus attention away from the Russian affair.
House Democratic Leader Nancy Pelosi said: "The Deflector-in-Chief is at it again."