Yesterday's tweet storm by the US president is still reverberating across Washington.

Donald Trump's accusations are astonishing. At the heart of them are claims that in the months before the election he was wire-tapped on the orders of Barack Obama.

He provided no evidence and Obama has emphatically denied it, although it is possible it was ordered by the FBI and authorised by a special court - under the Foreign Intelligence Surveillance Act (FISA) - that deals with classified intelligence matters.

In the latest developments on Sunday, James Clapper, the former Director of National Intelligence, strongly denied any suggestion that Trump Tower was being wiretapped before the election.

"There was no such wiretap activity mounted against the president-elect at the time, as a candidate, or against his campaign," Clapper told NBC.

At the same time, the President has demanded that Congress launch an investigation into his claims, even though he has provided no evidence. Critics regard that as an attempt to distract them from the main focus of the existing probe - finding out if Trump advisers and Russian officials were in secret contact.

Trump claims his campaign from Trump Tower in NY was being tapped. Credit: AP /Andrew Harnik /File

There are many questions that now emerge and which Congress will seek to clarify:

  • Is the central Trump allegation actually true? Did the agencies leading the counter-intelligence investigation into the Russian hacking operation seek to listen in to Donald Trump's phone calls?

  • Where did Trump get the information from? Did he base the explosive allegations about wiretaps on conspiratorial and incendiary articles on Breitbart and other fringe websites? Or did the president get the information from the intelligence agencies themselves?

  • Did the intelligence court - known as FISC - have evidence that Trump or his associates were in a relationship with Kremlin intermediaries? If not, and assuming they did issue a wiretapping warrant, what evidence did the FBI provide?

  • What next? Will President Trump back up his claims or retreat from them? Will Congress now launch new and more aggressive investigations into the whole complicated and secretive affair?

Even Republicans are stunned by what is developing. The Republican senator from Nebraska, Ben Sasse, put it this way: "We are in the midst of a civilization-warping crisis of public trust, and the president's allegations today demand the thorough and dispassionate attention of serious patriots."

The pressure is on Trump to provide evidence.

If he does not, then his presidency is once again in danger of being ridiculed by critics who are questioning not just Trump's political judgement but his mental and emotional health.