President Trump isn’t running for office in the U.K., but his presence is nonetheless looming over the country’s election, scheduled for Dec. 12.

British Prime Minister Boris Johnson, who currently leads a minority government which has struggled to pass crucial Brexit laws, is seeking a greater mandate to “get Brexit done.” If he returns as Prime Minister, he will also be tasked with securing a post-Brexit trade deal with the U.S.

Johnson enjoys a close relationship with President Trump, which he hopes will translate to a favorable trade deal further down the line. But their bond has also become a political liability during election time, thanks to Trump’s widespread unpopularity in the U.K.

While Britain may be divided on the question of Brexit, when it comes to Trump they broadly agree. In a poll conducted during Trump’s state visit to the U.K. in June, Gallup reported only 26% of British adults believed he was doing a good job leading the U.S. And in a Nov. 1 YouGov survey, only 10% of British adults said Trump’s endorsement would be helpful for party candidates hoping to win seats in the upcoming election.

So the main opposition Labour Party saw an electoral opportunity when, on Nov. 1, Trump called into a radio show hosted by the Brexit party leader Nigel Farage, and urged him to make an alliance with Johnson in pursuit of Brexit.

“You and I have become friends over the years,” Trump told Farage, who was one of the leading figures during the U.K.’s 2016 Brexit referendum, and visited Trump in New York shortly before his inauguration. “I would like to see you and Boris get together,” Trump continued. “I know that you and him will end up doing something that could be terrific… You’d be an unstoppable force.”

After his discussion with Trump, Farage announced that his insurgent Brexit Party would not field candidates against sitting Conservative lawmakers — a move which took some (though not all) of the pressure off Johnson.

Labour campaigners seized on the association. “What we have before us is an alliance between Donald Trump and Nigel Farage and Boris Johnson,” Labour leader Jeremy Corbyn told reporters on Tuesday. “We know where that alliance is designed to take us – into a sweetheart trade deal with the United States that will threaten all of our regulations, all of our conditions, and threaten our public services.”

After Brexit actually happens (it’s currently scheduled for Jan. 31, but has been delayed three times), the U.K. will have to hammer out a new trade deal with the U.S. — something Johnson has made a major priority of his post-Brexit economic strategy.

“Boris Johnson wants to be able to point to the possibility of a strong U.S.-U.K. trade agreement after Brexit, so he has to be on the same page as President Trump,” says Lewis Lukens, who was the most senior career diplomat based at the U.S. embassy in London from 2016 to 2019. “It helps Johnson to have positive encouragement on the trade front from President Trump. But on the other hand, a lot of people see President Trump pretty negatively in the U.K., and Johnson doesn’t want to be seen as too close to Trump. It’s a fine line to walk.”

Labour insiders say their campaign has been bolstered by anti-Trump messaging. They say the party has done research which shows the British population has widespread anxieties about, in particular, potential U.S. encroachment into the U.K.’s National Health Service (NHS) post-Brexit. (The NHS is an institution, beloved in Britain, which provides tax-funded healthcare free for patients to access.)

On his state visit to the U.K. in June, Trump played into those fears. “When you’re dealing in trade everything is on the table, so NHS or anything else… everything will be on the table, absolutely,” Trump told reporters at a press conference.

The Labour Party, which wants to remain in the E.U.’s economic unions (thus avoiding the prospect of a bespoke U.S. trade deal), has said that after Brexit, Trump will use America’s economic muscle to force the U.K. into accepting healthcare reforms that could push up prices for patients. On Facebook, many of the campaign ads taken out by Labour refer to Johnson’s perceived willingness to cooperate with Trump on this front. “Boris Johnson’s disastrous Brexit would sell off our NHS to Donald Trump,” says one advert, which first ran the day after Trump’s call with Farage, and stayed online for a week.

“With a No Deal Brexit, your family’s medicine costs could skyrocket to unaffordable USA prices once Boris Johnson strikes a trade deal with Donald Trump,” says another ad which ran on Facebook in October.

Johnson has repeatedly denied the NHS will be a part of any trade negotiations. But according to a recent report by Channel 4’s Dispatches program, senior officials from his government have met with U.S. negotiators and pharmaceutical companies to discuss the health service.

“The U.S. trade negotiators are very skilled and experienced and tough,” Lukens tells TIME. “They will be out there negotiating as hard as they can to protect U.S. interests. A lot of interests between the U.S. and U.K. are pretty well aligned, but there will be areas where what the U.S. team wants is not what the U.K. team wants. The negotiations will be difficult at that point.”

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Write to Billy Perrigo at billy.perrigo@time.com.