Nancy Pelosi has claimed William Barr has gone “off the rails”, after the attorney general alleged on Capitol Hill that US intelligence spied on members of Donald Trump’s presidential campaign.

In a further broadside amid efforts by Democrats to press Mr Barr to release a full copy of Robert Mueller’s investigation into Russia’s alleged interference in election and possible collusion with Mr Trump, the House Speaker said Mr Barr’s behaviour was dismaying and disappointing.

“Let me just say how very dismaying and disappointing that the chief law enforcement officer of our country is going off the rails yesterday and today,” Ms Pelosi told reporters at the House Democrats’ annual issues retreat at a Virginia’s golf resort near Washington DC. “He is the attorney general of the United States of America, not the attorney general of Donald Trump.”

Mr Barr, who has said he will provide legislators a redacted copy of the special counsel’s 400-page report, this week said he had started a probe to discover how the FBI began investigating members of Mr Trump’s campaign.

On Wednesday, testifying before the Senate appropriations subcommittee, he stirred things further when he said he believed that “spying on a political campaign is a big deal”.

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. The talks continued well into 2016 during the campaign, he has said. AP Mueller investigation: The key figures Stormy Daniels has alleged that she had an affair with Donald Trump in 2006, soon after Melania Trump gave birth to Baron Trump. The accusation is of particular importance as a result of the $130,000 hush money payment she received to keep quiet about the affair during the 2016 campaign. AP Mueller investigation: The key figures Paul Manafort was Donald Trump's former campaign chairman. Manafort was charged alongside Rick Gates for a slew of financial crimes, and was convicted on several counts in a Virginia court. He then pleaded guilty to separate charges filed in a Washington court. Manafort has been sentenced to just 7.5 years in prison for his crimes — in spite of recommendations from the special counsel's office for a much harsher sentence. AP Mueller investigation: The key figures George Papadopoulos was one of the first individuals associated with the Trump campaign to be charged by the Mueller probe. He ultimately received a 14 day prison sentence for lying to investigators about contacts he had with Russian officials. AP Mueller investigation: The key figures Roger Stone is a well known political fixer and operative, who has made a name for himself for some dirty tactics. He has been charged by the Mueller probe earlier this year, and he has been said to have had prior knowledge that WikiLeaks planned on publishing stolen emails from the Hillary Clinton campaign in 2016. Getty Images Mueller investigation: The key figures Rick Gates was charged alongside former Trump campaign chairman Paul Manafort for a range of crimes. Gates, who worked alongside Manafort for a pro-Russia Ukrainian political party. The two were charged with conspiracy and financial crimes. Gates pleaded guilty. AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

Asked to clarify his remarks, he said: “I think spying did occur. But the question is whether it was adequately predicated and I am not suggesting that it wasn’t adequately predicated.”

He added: “I am not suggesting those rules were violated, but I think it is important to look at that. And I am not talking about the FBI necessarily, but intelligence more broadly.”

Mr Trump has repeatedly claimed that his campaign was unfairly monitored during the 2016 presidential cycle, frequently voicing his frustration on Twitter. In March 2017, he wrote: “Terrible! Just found out that Obama had my “wires tapped” in Trump Tower just before the victory. Nothing found. This is McCarthyism!”

It is known that the FBI sought and obtained a warrant from a special Foreign Intelligence Surveillance Court, often referred to as a FISA warrant, to monitor Carter Page, the Trump campaign advisor who was suspected of having ties to Russia. CNN said although Mr Page left Mr Trump’s campaign in September 2016, after articles highlighted his Russian ties, the following month the FBI applied for a warrant to watch him.

The warrant was approved and subsequently renewed three times. Mr Page gave testimony on Capitol Hill and sent a letter to Mr Mueller’s team requesting information about the warrant, but never faced any charges.

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At the White House on Wednesday morning, Mr Trump claimed such surveillance amounted to “an attempted coup”.

“This was an attempted take-down of a president. And we beat them. We beat them. So the Mueller report, when they talk about obstruction we fight back. And do you know why we fight back,” he said.

“What I’m most interested in is getting started, hopefully the attorney general, he mentioned it yesterday.”

Speaking to reporters, Ms Pelosi, arguably the most powerful Democratic politician in the nation, doubled down on her party’s desire to see a copy of report of the special counsel.

“We want to see the Mueller report. There was an assault on the integrity of our election in our country, the basis for our democracy. There is no doubt about that. … It took place by Russia,” Ms Pelosi said, according to The Hill.

“You would think that every resource in our country would say we want to make sure this never happens again instead of engaging in this silliness. But, no, it isn’t silly; it’s too serious to be silly — this obstruction of getting the truth to the American people.”