London (CNN) Nine days before people in Britain head out to vote in the December 12 snap election, the nation's political leaders will come face-to-face with a US politician who has had an unusually high profile in this campaign.

Donald Trump will arrive in London on December 3 for a summit of NATO members. He will do so having already intervened personally in the election.

Earlier this month, Trump took the unprecedented step of saying not only that he would prefer Boris Johnson's Conservatives to win, but controversially suggested that Johnson should do a pact with the rival Brexit Party, led by Johnson's rival, Nigel Farage.

However, here in Britain, an endorsement from the US President is not always a good thing. He's very unpopular with large sections of the public -- and the politicians know it.

Ever since the campaign got underway, Trump has been used as a weapon by many of Johnson's opponents -- most notably the main opposition Labour Party.

The charge: Johnson is so hellbent on getting Brexit done and grasping the political trophy of a trade deal with the US , he's willing to sell parts of the beloved National Health Service to American companies.

The claim itself is spurious. Johnson is a man who has just struck a deal with Europe and appears to have shifted significantly toward a friendly relationship with the European Union being his priority. The £500 million figure is easy to pull apart for anyone with access to a smartphone. And Johnson has repeatedly insisted that the NHS will not be on the table in any trade deal with the US.

True or not, it's a message that taps into a bias many voters will already have. "Labour has a structural advantage on the NHS," said Rob Ford, professor in politics at the University of Manchester. "The public simply trusts the Conservatives less than Labour and believes that the Conservatives are more likely to privatize parts of the NHS."

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Making this election about the NHS helps Labour in more ways than simply hammering the Prime Minister. Labour leader Jeremy Corbyn has a confusing Brexit policy and doesn't cope well with questions on his plan. And as Ford explained, "Every voter who is talking about Boris selling the NHS to Trump is probably a voter who isn't talking about Boris getting Brexit done."

Another party using Trump as a weapon is the Scottish Nationalists. The anti-Brexit SNP's primary objective is securing another referendum on Scottish independence. They claim that voting SNP is the only way "to put Scotland's future in Scotland's hands -- not those of Boris Johnson, Trump and Farage."

An indication of the perceived effectiveness of using Trump as an attack line can be seen in an analysis of paid-for Facebook ads from Labour and other groups that oppose Johnson.

According to Rory Smith, a senior investigator at First Draft, a non-profit organization working to expose disinformation around the world, Trump is often mentioned in Facebook adverts that have been taken out in the United Kingdom. "You could argue that he is being weaponized by certain opposition parties and groups that support them, especially around key issues such as the NHS. Some of those adverts even go as far as saying 'Stop Trump from becoming the NHS puppet master ,'" he says.

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Paid-for adverts are commonly used as part of a microtargeted campaign to reach specific voter demographics. Using this strategy to get the NHS message across makes sense for Labour, who need to win over traditional Labour supporters who voted to leave the European Union in 2016.

This group differs from other Leave voters, in that though they share Euroskepticism, they "tend to care more about issues like the NHS than Brexit," explains Rob Ford.

Tying Johnson to Trump is a clever strategy beyond the issue of the NHS. Trump, contrary to his own claims, is "unpopular with almost every voter group in Britain, but especially so with wavering voters," according to Ford.

This means that every time Trump calls Johnson his friend, he gives opponents the chance to hammer home this line. And Trump's British "friends" seem wise to this. An advanced search of both the Conservative and the Brexit Party's Twitter feeds for posts containing the word "Trump" result in a grand total of zero.

Johnson has tried to walk a tightrope on Trump. For diplomatic reasons, it would be foolish to upset the world's most powerful man. However, when Trump came to the UK in June for his state visit, Johnson declined to meet with him in person. The visit happened during the Conservative Party leadership contest, and numerous sources involved in the campaign have explained to CNN that there was a general understanding that being seen alongside Trump might not help Johnson.

During the first televised debate between Johnson and Corbyn, both men were asked to name the world leader they most admired. Johnson declined the chance to cement his friendship with Trump, instead saying "the EU27. All of them, because they did me a fantastic deal." Fortunately for Johnson, Trump was probably glued to the TV and Twitter, monitoring his own domestic problems.

It's all but guaranteed that as the NATO summit and Trump's arrival approaches, Johnson's opponents will crank up the volume on the Trump alliance and how the two men will work together, wrecking the UK and the NHS.

Which leaves Johnson in a very tricky position. Assuming he attends the summit, he will have to walk that very same tightrope, only this time with the elephant in the room.

Fairly or unfairly, it will provide his opponents with an open goal and any photos of the pair together will doubtless feature heavily in their social feeds as polling day draws near.

All Johnson can do is pray that it doesn't blow up too bigly. And that the President doesn't give one of his now trademark interviews and, in doing so, hand Corbyn an early Christmas present.