President Trump accused former President Barack Obama of wiretapping the phones in Trump Tower prior to the election, a claim that Obama spokesman said was “simply false.”

Mr. Trump offered no evidence to support the claims as he tweeted Saturday that Mr. Obama “had [his] ‘wires tapped.’”

The accusations come as Trump faces mounting criticism for his administration’s alleged ties to Russian officials. Intelligence officials concluded last year that Russian hackers had exposed emails from Democratic National Committee members in an attempt to discredit former Secretary of State and Democratic candidate Hillary Clinton and to bolster Trump’s chances of winning the presidency. Obama responded by imposing sanctions on Russia and ousting Russian diplomats in December.

Since, Trump has pushed back against such claims that threaten his legitimacy as president, and tussled with Obama.

“How low has President Obama gone to tapp my phones during the very sacred election process,” the president wrote on Twitter. “This is Nixon/Watergate. Bad (or sick) guy!”

The allegations took Trump’s team by surprise, some of whom said they did not know if his comments referred to the investigation, or a new report.

A spokesman for Obama denied the claims.

"Neither President Obama nor any White House official ever ordered surveillance on any US citizen. Any suggestion otherwise is simply false," he said in a statement.

If true, the claims would stir seriously legal and ethical questions regarding the authority and actions of the former president.

Some raised concerns about the severe allegations.

"If there is something bad or sick going on, it is the willingness of the nation's chief executive to make the most outlandish and destructive claims without providing a scintilla of evidence to support them," Rep. Adam Schiff (D) of California said in a statement.

But many members of Congress on both sides of the aisle remained relatively quiet.

This isn’t the first time the president has made serious legal charges without citing evidence. He has repeatedly claimed that millions of undocumented immigrants illegally voted for Mrs. Clinton in the election, causing him to lose the popular vote. Officials and multiple studies have found no evidence of widespread voter fraud. Trump also falsely claimed for years that Obama was born outside of the United States, an assertion that, if it had been true, would have rendered his presidency illegitimate.

Some officials close to Trump said they believed he was referring to a Breitbart article published Friday.

In order to legally wiretap Trump and his campaign, a federal court would have to have ruled there was probable cause that he was “an agent of foreign power.” A White House official told The New York Times that Donald McGahn II, chief counsel to the president, was seeking to nail down an alleged order from the Foreign Intelligence Surveillance Court that would’ve authorized such action.

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There is not currently any publicly available evidence that such an order exists.

This report contains material from Reuters.