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.

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 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."

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."

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.

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Donald Trump is unpopular in Britain. That's why he's being used to attack Boris Johnson - CNN International