LONDON — Donald Trump lobbed a grenade into the heart of British politics Thursday night, something that's become a habit for the president, as he called into a radio show hosted by one of his friends on the populist right.

In his call with leading Brexit supporter Nigel Farage on LBC radio, Trump praised Prime Minister Boris Johnson as a "fantastic guy" and said opposition Labour Party leader Jeremy Corbyn would take the country "into such bad places."

But because Trump is reviled by so many in the U.K., his intervention might have the opposite effect of his stated intention. Just 19 percent of Brits have a positive opinion of Trump, according to the pollster YouGov, and more than two thirds say they have no confidence in the president to do the right thing, a study by the Pew Research Center found last year.

Britain just kicked off a divisive and unpredictable campaign ahead of a rare December election. The contest ultimately seeks to answer how the country should leave the European Union, or whether it wants to at all.

Nigel Farage joins then presidential candidate Donald Trump at a campaign rally in Jackson, Mississippi, in August 2016. Carlo Allegri / Reuters file

Trailing in the polls, the opposition Labour Party immediately turned the president's negative remarks into a campaign line.

It was a chance for Corbyn, a left-wing firebrand, to promote himself as Trump's ideological opposite, while casting Johnson as the bedfellow of a president widely disliked in the U.K.

"Donald Trump is trying to interfere in Britain's election to get his friend Boris Johnson elected," Corbyn tweeted within minutes of the interview being aired.

Donald Trump is trying to interfere in Britain’s election to get his friend Boris Johnson elected.



It was Trump who said in June the NHS is “on the table”. And he knows if Labour wins US corporations won’t get their hands on it.



Our NHS is not for sale.pic.twitter.com/AUhht3pCgL — Jeremy Corbyn (@jeremycorbyn) October 31, 2019

And although Trump praised Johnson, as he has done in the past, in the next breath he undermined the prime minister's core Brexit policy. Trump said the deal Johnson negotiated with the E.U. would make striking a separate agreement with the U.S. impossible.

"To be honest with you, under certain aspects of the deal … you can't trade," Trump said without going into specifics. "I mean, we can't make a trade deal with the U.K."

Johnson is the second British prime minister to be rocked by a surprise intervention from Trump. The president humiliated Johnson's predecessor, Theresa May, lambasting her Brexit strategy in July last year, hours after she had rolled out the red carpet for him in London.

A sign of how damaging these presidential forays can be, the British government was forced to respond Thursday and deny that Johnson's Brexit deal precluded one with Washington.

On the other end of the spectrum, rather than harming Corbyn, Trump's call was a "gift" to the Labour leader, tweeted Theo Bertram, a former adviser to Labour prime ministers Tony Blair and Gordon Brown.

Johnson meets Trump in Biarritz, France, this August. Stefan Rousseau / AP file

In wading into British politics, Trump touched on one of Labour's central campaign messages: protecting the country's beloved and free-to-access National Health Service.

Labour alleges the Conservatives want a post-Brexit trade deal with the U.S. that would allow American firms to do harm to the NHS. Johnson has repeatedly denied this, and on Thursday Trump did, too.

"It's not for us to have anything to do with your health care system," he said.

However, this summer the president and his ambassador in London both said that the NHS would be "on the table" in any post-Brexit trade deal, albeit rowing back the comments after backlash from horrified Brits.

Furthermore, U.S. trade objectives appear to advocate giving American pharmaceutical giants more control over drug prices in the U.K., something currently regulated by a British government body.

No matter that Trump denied ill intent, his very mention of the NHS "guarantees this as an election issue" for Labour, Bertram, the former government adviser, said.

Another potential winner from the exchange is Farage himself.

Trump told Farage that he and Johnson should "get together" and unite as an "unstoppable force" in the election.

In doing so, the president boosted the status of his ally who, despite leading the Brexit charge in 2016, has failed seven times to become an elected lawmaker in London.

Farage heralded his interview with Trump as a major scoop. This has drawn accusations of hypocrisy, given he said that President Barack Obama "behaved disgracefully" for intervening during the 2016 Brexit referendum.

"You wouldn't expect the British prime minister to intervene in your election," he said back then. "You wouldn't expect the British prime minister to endorse one candidate or another."

It's unclear how the Conservatives and Farage's insurgent newcomer Brexit Party will affect each other during the campaign. The Conservatives lead the polls by 10 percentage points or more, but Farage's group, polling at about 10 percent or better, threaten to split the right-wing vote.

Farage claims Johnson's deal is not a pure enough form of Brexit, and has suggested a pact if the government drops this marquee policy.

This has been rejected by the Conservatives, perhaps wary of being associated with a man accused of using xenophobic and racist language and imagery.

Will such an alliance appearing unlikely, Farage told a campaign launch Friday that "the Brexit Party will be the only party standing in these elections that actually represents Brexit."