The "Deep State" is a concept which has been used by some vocal supporters – and opponents – of Donald Trump.

Sometimes also referred to as the "Permanent State", the theory espouses the idea that a cabal of unelected security officials across a number of government bodies maintain influence over elected politicians.

It has a historic role in the political consciousness of the US, first gaining prominence during the 1960s following events like the assassination of President John F Kennedy. It is now regaining popularity.

Mr Trump's supporters referenced the "Deep State" after his former National Security adviser Michael Flynn was forced to resign, after it emerged that he had discussed lifting sanctions on Russia, with the country's ambassador Sergey Ivanovich Kislyak, before taking office. It is illegal for a private citizen to conduct diplomacy on behalf of the US.

But some of Mr Trump's supporters were suspicious of the insider leaks from security officials that prompted him to quit.

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Mr Trump has also had to deal with allegations against Attorney General Jeff Sessions, after he reportedly lied at his confirmation hearing about his contact with Russia.

Shortly afterwards, Mr Sessions recused himself from any investigation into any Russian interference in the 2016 Presidential elections.

The report which led to the damaging allegations was based on anonymous sourcing from within the intelligence community.

This led to further talk from some supporters of Mr Trump of the "Deep State" attempting to sabotage the Presidency.

Believed to have some basis in reality in despotic countries like Turkey and Egypt, observers say the "Deep State" has made itself known at times of great tension.

Last summer’s attempted coup in Turkey and the 2013 ousting of the democratically elected Egyptian President Mohamed Morsi, are among recent examples of this occurring, some commentators have claimed.

But in the modern US, where democratic institutions and traditions are far stronger, the "Deep State" has generally been the realm of conspiracy theorists. No solid evidence has been produced for its existence.

“The notion that there’s a grand conspiracy which involves state and non-state actors such as the oil and defence industries I think is pushing the boundaries of credibility,” Dr Paul McGarr, Assistant Professor in US Foreign Policy at the University of Nottingham, told The Independent. “I don’t think there’s any evidence which has ever been produced to substantiate those claims”

For one intelligence official, the concept was laughable. “The deep what?” they told Buzzfeed News.

Despite this, Mr Trump himself has alluded to the theory, according to Dr McGarr and other observers.

The US President is believed to be a reader of outlier news organisations which regularly mention the concept.

After Mr Flynn’s resignation, Mr Trump tweeted: “The real story here is why are there so many illegal leaks coming out of Washington? Will these leaks be happening as I deal on N.Korea etc?”

“It goes back to this idea that Nixon and certain Presidents of the right have regarded the CIA – despite the popular image of the agency being reactionary – as an Ivy League, liberal bastion of counter reaction against the right,” Dr McGarr said. “This goes back to the foundation of the CIA in the 1940s and a notion that they are liberals and against the right wing. Nixon tapped into that and I think Trump is doing the same.”

Observers say this is also feeds into why the concept has regained popularity - in line with the more seismic changes in America.

The anti-establishment nature of Mr Trump’s campaign – so loved by his supporters -- fed into a notion of "America embattled".

“As political ploy it’s incredibly effective,” said Dr McGarr. “It’s something that can explain why blue collar workers have lost their jobs and taps into the 'white fear'”.

Dr McGarr said the notion of a “state within a state” has a powerful hold, not just over the popular imagination, but with the press too.

Mr Nixon felt he lost the election against John F Kennedy in 1960 because of leaks from the intelligence establishment and maintained a grudge against the CIA, Dr McGarr said. He apparently regarded them as having interfered in the political process – just as Mr Trump is now.

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“Lots of the interesting noises Nixon made about ‘constraining the CIA’, ‘reigning them in’, is very similar language to what Trump is using at the moment,” Dr McGarr added.

The idea of a ‘Deep State’ has been touted across conservative media outlets. In particular, it has been referenced dozens of times by provocative, right wing news website Breitbart, who referred to the latest round of leaks as "DeepStateGate".

The website’s controversial founder, Steve Bannon, now sits at the heart of power in the White House.

“Breitbart does seem to have a surprising degree of influence over Trump,” Dr McGarr said. There have been suggestions the President's tweets accusing his predecessor, Barack Obama, of wiretapping Trump Tower may have been prompted by a Breitbart story.

“Bannon is certainly a tremendously influential figure in the White House and he does seem to be wedded to this idea of conspiracism,” Dr McGarr said. “I think that’s got to, at some point, feed through to Trump himself”.

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Others have pointed out how leaks from the intelligence community differ from a secretive attempt to sabotage the government.

“The level of anxiety in the [national security] community is at the highest level I’ve experienced in my professional life,” Peter Feaver, former Bush administration official and professor at Duke University, told Vox. “Desperate times call for desperate measures.”

Despite the questionable place of the "Deep State" in politics, Dr McGarr believes it could be around for some time yet.