President Donald Trump's arrival in the U.K. just days before Britons head to the ballot box could spell trouble for Prime Minister Boris Johnson, political analysts told CNBC. The U.S. president, who landed in London on Monday evening for a NATO summit marking the alliance's 70th anniversary, is likely to be pressed to comment on a range of hyper-sensitive political issues — ranging from the popular National Health Service to prospective trade talks. Britons will cast their ballots on Dec. 12 in a vote likely to decide the fate of the U.K.'s departure from the European Union and the direction of the world's fifth-largest economy.

Johnson has urged Trump not to get involved in the election, fearing he could say something that threatens to derail the Conservative Party's campaign. "What we don't do traditionally as loving allies and friends, what we don't do traditionally, is get involved in each other's election campaigns," Johnson, who's center-right party holds a commanding lead in the latest opinion polls, told LBC radio on Friday. The prime minister also said he would walk out of trade talks with the U.S. if the health service was a precondition to negotiations.

Beware of the 'bogeyman'

Trump has typically been unafraid to comment on a wide range of U.K. election issues, with Labour Party leader Jeremy Corbyn often quick to use the U.S. president's praise of Johnson as a key attack line against the Conservatives. In recent months, Trump has suggested Johnson and Brexit Party leader Nigel Farage would form an "unstoppable force" if they formed a cross-party alliance, claimed Corbyn would be "so bad" for Britain and indicated the NHS would, in fact, be "on the table" during prospective trade talks. He has even warned that Johnson's Brexit deal might ultimately block a much sought-after trade deal between the U.S. and the U.K. "He's a bit of a bogeyman," Matthew Oxenford, lead U.K. and Brexit analyst at the Economist Intelligence Unit, told CNBC via telephone.

Opposition Labour party leader Jeremy Corbyn (C) poses with NHS workers holding documents regarding the Conservative government's UK-US trade talks in London after a press conference on November 27, 2019. TOLGA AKMEN | AFP | Getty Images

A lot of external commentators have offered their opinion on U.K. politics in recent weeks, Oxenford said, citing the outgoing European Council President Donald Tusk and former U.S. presidential candidate Hilary Clinton. But, when it comes to Trump, the "biggest issue" was clearly likely to be the NHS, Oxenford said. On a state visit to the U.K. earlier this year, the U.S. president suggested that the NHS — which has provided free health care at the point of use for more than 70 years — would be up for grabs during post-Brexit trade talks. Standing alongside then-Prime Minister Theresa May in June, Trump told reporters: "When you're dealing in trade everything is on the table. So, NHS or anything else, or a lot more than that — but everything will be on the table, absolutely."

An overwhelming majority of Brits reject his politics, even as many realize that Britain without the United States is far weaker than with the United States in a 'Special Relationship.' Brian Klaas Professor of global politics at University College London

Trump has since sought to backtrack on his comments about the NHS, saying last month that health would not feature in post-Brexit trade negotiations. "Sometimes he says it will be on the table in trade talks, sometimes he says it won't be on the table. This confusion gives Labour an opportunity to use it as a good attack line," Oxenford said.

Key battleground

Corbyn has warned that a Conservative-led trade deal with the U.S. could drive up the price of medicines, warning Johnson's party would sell off parts of the health service to U.S. business after Brexit. Johnson has repeatedly insisted the state-run NHS would not be on the table in any trade talks. "Trump doesn't have a filter. It won't matter if his advisers tell him — or Boris pleads with him — not to speak about the election, the NHS, or what Britain will have to give up to the United States in a prospective trade deal," Brian Klaas, a professor of global politics at University College London, told CNBC via email. "He also has a very limited understanding of British political sensitivities. In previous visits, it wasn't clear that he knew what the NHS was, let alone how sensitive it is for an American president to speak about it," he added.

President Donald Trump and Britain's Prime Minister Theresa May hold a joint news conference in London, Britain, June 4, 2019. Carlos Barria | Reuters

The NHS is often a key battleground during an election period, with politicians quick to champion an institution that is celebrated across the country. However, despite its recognition as one of Britain's most cherished institutions, problems associated with the NHS remain a regular feature of public discourse. This includes near-constant concerns about a shortfall in funding, long waiting times for consultations and operations, cutbacks to social services, crumbling hospitals and staff shortages. "Trump is, in general, a toxic political force in the U.K. An overwhelming majority of Brits reject his politics, even as many realize that Britain without the United States is far weaker than with the United States in a 'Special Relationship,'" Klaas said.

US-UK trade talks

Oxenford said that given the U.S. president was "never one to refrain himself," he will "undoubtedly" wade into the U.K. election campaign during his visit to London. "He is generally an unpopular figure in the U.K. so Johnson won't want to embrace him — but he can't ignore him either." U.S. trade negotiators have set out their objectives for talks with the U.K., including "full market access" for U.S. pharmaceutical companies.

U.S. President Donald Trump and Britain's Prime Minister Boris Johnson arrive for a bilateral meeting during the G7 summit on August 25, 2019 in Biarritz, France. Stefan Rousseau | Pool | Getty Images