LONDON — During previous elections in the United Kingdom, political parties have actively courted the endorsement of the president of the United States.

But Donald Trump, in town for a NATO meeting on Tuesday, is so unpopular in the U.K. that his traditional enemies are hoping he will say something — anything — to denigrate them or the causes they support.

The president's visit comes ahead of a pivotal nationwide election on Dec. 12 that could shape the U.K.'s Brexit path for decades to come.

"Obviously he is very unpopular with British people, and I don't think he does Boris Johnson any favors," said Matt Zarb-Cousin, a former spokesman for opposition Labour Party leader Jeremy Corbyn, who is seeking to unseat Johnson as prime minister.

"I don't think Trump is proficient enough not to land Boris Johnson in it by accidentally saying something" damaging about the National Health Service, Zarb-Cousin added, referring to Britain's publicly funded health care system, which has become a central issue in the campaign.

Brits have reason to expect fireworks.

During past visits, Trump has made controversial comments about the NHS, repeatedly insulted London's mayor, Sadiq Khan, and given a surprise newspaper interview humiliating his host, then-Prime Minister Theresa May.

Meanwhile, just 19 percent of Brits have a positive opinion of Trump, according to the pollster YouGov. More than two thirds say they have no confidence in him to do the right thing, a study by the Pew Research Center found last year.

Johnson, who Trump calls a "good friend," says he will not be holding a formal one-on-one meeting with the president this week.

Johnson's aides have denied the suggestion that he fears any association with Trump would hurt his campaign. Polls show his Conservative Party is in the lead, but the gap with Labour has narrowed in recent days.

So it may not be a surprise that Trump cut a more neutral tone this time.

"It's going to be a very important election for this great country," the president told reporters alongside Jens Stoltenberg, NATO's secretary general, on Tuesday. "But I have no thoughts on it."

Later he added, "I stay out of it — I think Boris is very capable and will do a good job."

Some hope that Trump will yield something that can be weaponized for the election.

"This guy is verbally incontinent," journalist and left-wing activist Paul Mason said on BBC television. "There's many hours yet to go. And remember he's jet-lagged so there's plenty of time when he's going to be awake, watching Fox News and reacting to what's happening in the world."

Today marks a stark contrast to 2009, for example, when then-Prime Minister Gordon Brown tried five times before successfully securing a meeting with President Barack Obama.

Back then, Brown was the centrist leader of Labour. Corbyn is a veteran socialist campaigner who casts himself as Trump's antithesis.

Central to his anti-Trump stance is the NHS, which is publicly funded and provides free care for everyone at the point of use.

After the U.K. leaves the European Union, Washington and London want to strike a new trade deal. One of Washington's aims is "free market access" for drugs sold by American pharmaceutical companies, according to trade objectives published in February.

This summer Trump said that the NHS would be "on the table" in any post-Brexit trade deal, albeit rowing this back after widespread outrage in Britain.

On Tuesday, Trump doubled down on this denial, saying he wouldn't meddle with the NHS even if it was handed to him "on a silver platter." He said he didn't know where the "rumor" about the NHS being involved in the trade deal came from.

Asked why he was holding back on commenting on the election, Trump appeared to acknowledge his widespread unpopularity across Europe.

"I'm representing the U.S. So they may not like me because I’m representing us, and I represent us strong," he said. He also claimed he "knows nothing" about Corbyn, a departure from comments in October when he said the Labour leader "would be so bad for your country, he'd be so bad, he'd take you on such a bad way. He'd take you into such bad places."