An FBI agent has surrendered his badge over what he described as the politicization of the bureau amid the furore of a controversial memo made public by Congress.

Josh Campbell, once special adviser to the director, has resigned in order to have his say over the concerns he has without fearing the backlash his opinions might cause.

The FBI is reeling after the Trump administration went public with the document, which claims to show the bureau colluded with the Democrats to gain information which would allow agents to spy on the Republican campaign party during the 2016 election race.

Democrats have chimed in on the argument over the weekend, calling it a 'political hit job' and an attempt to 'undermine and discredit the FBI'.

But President Trump claimed the document vindicated his insistence there was no collusion with the Russians and no obstruction during the race to The White House.

Josh Campbell, once special adviser to the director, has resigned in order to have his say over the concerns he has without fearing the backlash his opinions might cause

Campbell, who wrote a column for the New York Times, said his former colleagues at the bureau were 'dogged', a-political people who needed the pubic's backing.

'Scorched-earth attacks from politicians with partisan goals now threaten that support, raising corrosive doubts about the integrity of the FBI that could last for generations,' he wrote.

Campbell was an agent with the FBI for more than a decade, and said it was one of the greatest honors of his life.

Campbell was an agent with the FBI for more than a decade, and said it was one of the greatest honors of his life

He worked on counter-terrorism, was part of kidnapping rescue teams and was a special assistant to the director during his time.

But he said he felt forced to turn in his badge to join the 'growing chorus of people' who believe the 'relentless attacks on the bureau undermine not just America’s premier law enforcement agency but also the nation’s security'.

'My resignation is painful, but the alternative of remaining quiet while the bureau is tarnished for political gain is impossible,' Campbell wrote.

He goes on to say that agents at the FBI make mistakes like any human, but that this is not one of them.

These political attacks on the bureau must stop,' he said.

'If those critics of the agency persuade the public that the FBI cannot be trusted, they will also have succeeded in making our nation less safe.'

Campbell wasn't out of work for long, having joined CNN as a law enforcement analyst and will appear on air to discuss crime, justice, and national security issues.

Donald Trump Jr has described the releasing of the controversial FBI memo 'sweet revenge' for him and his family.

His father President Trump claimed the document vindicated his insistence there was no collusion and no obstruction during the 2016 election.

Trump Jr, talking after the memo was made public over the weekend, said he was pleased the information had come out, saying the only collusion had been at the top of end of the FBI.

He accused the FBI of spying on the Trump campaign in an attempt to prove the Republicans were working with the Russians.

Donald Trump Jr has described the releasing of the controversial FBI memo 'sweet revenge' for him and his family

President Trump claimed the document vindicated his insistence there was no collusion and no obstruction during the 2016 election

Talking to Fox News, he said document's release was a good thing because it meant the alleged practices had been highlighted.

'It's pretty incredible,' he said.

'What I want people to do is think about what if Bush had done this to Obama - what would the reaction be?'

Trump Jr said the Democrats worked with the FBI and Department of Justice (DOJ) to attain surveillance warrants which would allow agents to spy on his father's campaign team.

'They weaponized the FBI and the DOJ to attack the duly-elected President of the United States,' he said, talking of Hilary Clinton and Barack Obama's officials.

Dick Durbin, the number two Senate Democrat, warned on Sunday against removing senior officials

Democrats on Sunday sharpened their rhetoric in the clash over the investigation into Trump campaign links to Russia, warning the president that firing top law enforcement officials could 'lead to a confrontation we do not need in America'.

The warnings came two days after the Republican-led House intelligence committee released a declassified memo that claims that Democrat-funded research prompted the FBI to spy on a former Trump campaign aide, Carter Page.

Democrats stoutly resisted the memo's release, saying it was misleading, and possibly intended to lay the groundwork for President Donald Trump to fire senior officials and subvert the Russia meddling investigation led by Robert Mueller.

Dick Durbin, the number two Senate Democrat, warned on Sunday against any such move.

'The question at that moment is whether or not the majority Republicans in the House and the Senate will stand up for the rule of law and the Constitution if the president takes that extreme position,' Durbin said on CNN's State of the Union.

'If the president takes this extreme action, I'm afraid that it could lead to a confrontation we do not need in America.'

Both he and the ranking Democrat on the intelligence committee, Representative Adam Schiff, contested the suggestion that the FBI's request to a special court for authority to surveil Page failed to note that it was based in part on a dossier paid for by Democrats.

'The court was notified there was a political actor involved,' Schiff said.

'The goal here is to undermine the FBI, discredit the FBI, discredit the Mueller investigation, do the president's bidding,' he said on ABC's 'This Week.' He said it was 'very possible' committee Republicans had 'coordinated the whole effort with the White House.'

Talking of the Democrats' alleged involvement, Trump Jr said: 'What I want people to do is think about what if Bush had done this to Obama - what would the reaction be?'

Trump tweeted Saturday that the memo 'vindicated' his insistence that there was 'no collusion and there was no obstruction.'

But some Republicans, while supporting the gist of the memo, took pains to show support for Mueller, deputy attorney general Rod Rosenstein and FBI chief Christopher Wray at a time the FBI and Justice Department are facing almost unprecedented criticism from the president.

'I've had my differences with Rod Rosenstein, and I still think that he is fully capable of helping run a Justice Department that we can all have confidence in,' said Representative Trey Gowdy, chairman of the House Oversight Committee, on CBS's 'Face the Nation.'

And Representative Will Hurd, a Republican on the intelligence committee, told ABC, 'I want to stress Bob Mueller should be allowed to turn over every rock, pursue every lead so we can have trust knowing what Russia did or didn't do.'