When Donald Trump was in Britain for NATO's 70th anniversary summit last week, U.K. Prime Minister Boris Johnson had to walk a fine line between maintaining his relationship with the U.S. president while avoiding been seen as too close to him.

With his mop-like hair and populist streak, Johnson is often compared to the U.S. president, usually by the prime minister's enemies in an attempt to taint him by association — Trump is extremely unpopular in the U.K. with just 18% having a positive opinion of him, according to recent YouGov data.

Some of the comparisons are valid. Both men have courted working class, or blue collar, voters with anti-immigration rhetoric; both have bent the truth to achieve their goals — Johnson has been fired for lying twice in his career, once as a journalist for making up a quote and once as a shadow minister for lying about an extramarital affair.

In other ways, they are quite different. When Johnson was mayor of London between 2008 and 2016, he was a moderate who invested in housing and public transport as well as giving amnesties to illegal immigrants. He claims to be a One Nation Conservative, a more paternalistic ideology that aims to support all classes and is in many ways to the left of the U.S. Republican Party.

But just one day before the U.K. votes for a new government, it isn't clear what kind of prime minister Johnson would be, even as polls suggest he is headed for a majority.

Johnson has run his campaign on a message of "Get Brexit Done," promising to get the deal he has agreed with the EU passed by Parliament and secure a trade agreement by the end of 2020.

Beyond that, the Conservative manifesto is thin on detail.

A casual observer could be forgiven for thinking that his government is planning on a big boost to spending, with his promises of increased spending on the National Health Service, including new hospitals, thousands more nurses, and a big rise in police numbers.

However, the manifesto outlines an increase in spending on public services of just £14 billion by 2023-24, compared with £231 billion by the opposition Labour Party.

"The Conservative manifesto is remarkably brief on real change in either the tax or the spending system," said Paul Johnson, director of the Institute for Fiscal Studies.

Even on Brexit, the way forward isn't clear. Johnson's goal of securing a trade deal with the EU by the end of next year is by most estimations a highly ambitious timetable and completely unrealistic according to many. If it isn't done by then, some in his party will be pushing for the U.K. to leave without an agreement, falling back onto WTO terms, while others will want an extension of the transition agreement.

Johnson refuses to be drawn on the details, insisting only that he will "get Brexit done" and pass the "fantastic" deal he has agreed with the EU.

Brexit provides some other insights into what kind of politician Johnson is.

While a leading figure in the Brexit campaign, it was only at the last minute that he decided to join the Leave camp and has previously been a robust defender of the EU.

After promising the Northern Irish unionist party, the DUP, that his deal with the EU would not include a border with Great Britain through the Irish Sea, Johnson negotiated a deal that does just that, leading to a rift between the two traditional allies.

He has since gone on record to say that there will be no customs checks on goods passing from Great Britain to Northern Ireland, contradicting a report leaked from his own Treasury department.

It is in regard to his honesty where many of the comparisons with Trump, a serial purveyor of false statements according to fact checkers, have been made.

"I think both of them have a certain, rather elastic relationship with the truth," former Conservative Chairman Lord Patten told the BBC last week. "I think that, for them, the issue is not governing, it's campaigning. Mr. Trump is campaigning the whole time and I think Boris campaigns most of the time without thinking about what the consequences of what he says may be."

During the Brexit campaign, Johnson trumpeted free trading principles and the image of a buccaneering Britain reclaiming the global trading position it enjoyed in the 19th century. But this policy is at odds with a speech he made Nov. 29 to push for looser state aid rules when the U.K. is outside of the European Union, which would allow the government to step in and support struggling companies.

But while it is not clear whether Johnson will continue to embrace the nationalists who helped bring him to power, Trump is more single minded in that regard.

Trump's agenda has been dominated by the ambition to "Make America Great Again." This consisted of plans to regenerate the manufacturing sector by using tariffs to block imports, and rip up international agreements which he saw as harming the competitiveness of U.S. companies, notably the Paris agreement on climate change and the NAFTA trade deal with Canada and the Mexico. Further efforts to bring Iran and China to heal through sanctions and tariffs rounded off Trump's view that the U.S. was giving the world a free lunch.

Johnson has been far more supportive of global institutions. His championing of the World Trade Organization contrasts with Trump's desire to beat multilateral organizations into submission, while Johnson is at odds with Trump over the merits of NATO with the U.S. president repeatedly complaining that other members are not pulling their weight.

A key plank of Johnson's Brexit sales pitch is that the U.K. will be able to do trade deals with other countries, most notably the U.S., the world's largest consumer market, so his relationship with Trump is paramount.

Still, he has shown some willing to favor policies likely to grate the U.S. president.

Ahead of last week's NATO gathering in London, Johnson pledged to impose a digital services tax on the tech giants. The tax, which would amount to 2% on U.K. revenues, is similar to the 3% proposal of France's President Emmanuel Macron which led to Trump announcing 100% tariffs on some $2.4 billion of French goods.

One thing Trump and Johnson will hope to keep in common is their success at the ballot box.

In his role as chief Brexiter, Johnson mirrored Trump in galvanizing the populist vote, and it appears he may be able to repeat the trick in the Dec. 12 general election.

The latest polling from YouGov suggests the Conservatives have 43% of the vote versus 34% for Labour, enough to give Johnson a narrow majority of 28 seats in parliament.