Traditionally, a backdoor is something you would probably only hear about if you were in a discussion about buildings, cyber security, or basketball. I would like to argue why we should extend this to include politics.
The important question
Some believe the election of president Trump in 2016 was only possible because of the interference from foreign forces. Whether this is true or not, it doesn't change the more important question:
Could foreign forces influence a general election at all?
In the off-chance it wasn't possible before the election, it will be today - at least in western society.
"Normally" if some Super Power wanted to influence an election, the methods they had available might have included: bribes, assassinations and hacking of ballot voting systems. Maybe some governments still find these methods to be acceptable in some instances, but the problems with these methods are traceability as they can leave quite a few breadcrumbs, so it's a lot more work to prepare and cover up. So which methods do we have which are more suited towards influencing a super power's general election? The "normal" routes might not be as appealing here, as the risk of getting caught are much higher as it could lead to increased political tension, sanctions or in worst case maybe even war.
Assuming bribes, assassinations and hacking is out of the question, what options do you have left?
Ideology, the backdoor to our society
Its all about intentions and pandering to the narrative.
It has been demonstrated, both by the people of the far-left and law enforcement, that your actions do not matter so long they are justified by the end goal. So if foreign forces wanted to influence the political affairs of a country, they have almost free roam as long as they mask their actions with the far-left's intentions.
With social media connecting just about everyone, you don't even need to physically set foot inside the country you are trying to influence. You have access to the eyes and ears of citizens and politicians, from just about any means in which you can connect to the internet.
This applies to any politically extreme group who is against free speech and for a totalitarian state. As history shows the political climate changes from one side to another like a pendulum swings, so today its the far-left, but next time it could be the far-right. Whenever we have one extreme, it tends to breed another opposite extreme. Without critical thinking, and without learning from history there is no hope of us ever affecting the political pendulum.
No need for a trojan horse when the door is open
Because the far-left is so open and welcoming to any foreign influence, as one of the main point of their ideology is to literally be all-inclusive with no reservations, none of it's followers would know or even want to know if they were already infiltrated. There is no compromise there is only the end goal and these people will do whatever it takes to get there, no matter the cost of integrity, moral or the cost of others - as they have and keep proving.
Worst case scenario, if no one dares to criticise and those who do are censored or even prosecuted, I would not be surprised if the right combination of foreign influence could turn the far-left ultra progressives into a form of militia for another foreign power, without them or us even realising this. This may sound absurd, but it would not be the first time in history where censorship, fear and propaganda would allow for the uprising of something similar or even worse.
 This is sadly already happening in the UK, with the help of the communications act 2003, 127(1)(a).
With the far-left being vocal enough that they are able to pressure platforms into de-platforming people, getting people fired from their jobs and even in a disturbing number of cases, pressurising law enforcement into prosecuting anyone who criticises their views or they find "offensive". This means people are self-censoring, and stops criticising or even just questioning what they are being told by the supporters of the far-left and the public media supporting them. People take whatever they are told for granted.
I personally believe, the media lost their marbles with the Charlie Hebdo terror attack, since that attack they all appear to be in a completely submissive state where they do not dare to write any articles or criticises anything or anyone who could be offended. There is no doubt in my mind that the Charlie Hebdo terror attack has been the most successful attack in western society, as it has done exactly what it intended - to change the way we live our lives, and make us live in fear.
Preventing foreign influence
We should allow influence of other cultures and views, it teaches us much and allows us to better understand our differences, which could reduce political and personal tension which is derived from lack of understanding the other cultures. However we must acknowledge that not all influence is created equal, or have well intentions for the receiving end, which is why we must be critical.
We have inadvertently created a backdoor for foreign influence into the backbone of our society, and by extension our politicians and lawmakers. Without free speech and without the right to question everything, we would have no way of preventing foreign forces form influencing any part of our society negatively.
The far-left's good intentions created the backdoor, and the only thing we have standing guard is free speech, and free speech is currently on life-support.
A discussion between Tim Pool and Joe Rogan on the Joe Rogan Experience on the 8th of Feb 2019, talks about the issue with foreign and political influence, from a different angle but in much greater detail and much better explained (mostly in the last hour of the show):