LONDON — Britain is in the midst of a general election that all sides agree is one of the most important in a generation. But as candidates in 650 constituencies go about knocking on doors, the traditional method of campaigning in this country, they are doing so in a climate that is increasingly hostile to politicians.

This is the first December election in more than a century — Brits head to the polls on Dec. 12 — and it gets dark early here, shortly after 4 p.m. Some candidates are concerned about what they might encounter on the doorstep at a time when emotions around Brexit are running high.

The myth of absolute British politeness at all times has been punctured in recent months, with harsh language coming right from the top. In a particularly noxious parliamentary debate in September, Prime Minister Boris Johnson was widely criticized for using words such as “betray” and “surrender” to describe the actions of his rivals.

There have been signs of tension on the campaign trail. In the past week alone, a Labour Party volunteer in her 70s was thrown over the hood of a parked car in Herefordshire, and a 72-year-old Labour campaigner in South Yorkshire was taken to hospital with a suspected broken jaw.

Steffan Aquarone, who is running as a candidate for the Liberal Democrats in Norfolk, said in an interview that he recently knocked on a door at dusk and was told by a man that he would shoot him if he became a member of Parliament.

When the “gentleman who wasn’t very gentlemanly” made the remarks, Aquarone said it felt “personally threatening.”

“Because we’ve been requested to report any incidents like this to the police, I did. It actually hit me about 20 minutes after it happened, and I was quite shaken by this,” he said. (The next day, the “gentleman” reached out to apologize.)

For the first time in an election campaign, the National Police Chiefs’ Council issued security guidance for candidates, which recommend not to campaign alone and to keep records of any intimidating behavior or abuse.

For many candidates on the campaign trail, the murder of Jo Cox just days before the 2016 European Union referendum remains on their minds. The pro-E.U. Labour lawmaker died after she was repeatedly shot and stabbed on the street.

Cox’s sister, Kim Leadbeater, said in an interview that after her sister died, politicians promised to treat each other better, “but that didn’t last very long — over the last three years, it’s got progressively worse.”

“I’ve spoken to many MPs over the last three years who have had horrible abuse and intimidation, online and offline,” she said. “One MP was buying shampoo in the supermarket and was shouted at down the aisle, some have had their office windows smashed, some are out with children when they are shouted at. … I don’t want to be accused of scaremongering, but these are real things.”

In the wake of Cox’s death, the Metropolitan Police Service set up a special investigation squad to focus on crimes against lawmakers, staffers and families. In the year after Cox’s death, the unit received 111 reports of criminal threats; in 2018, it received 242, more than double. Cressida Dick, commissioner of the Metropolitan Police, has described the levels of abuse directed at lawmakers as “unprecedented” and a result of Britain’s growing polarization.

The abuse is not limited to one side of the Brexit debate, or to men or women. But researchers say that women and ethnic minorities are disproportionately targeted. In the last election, Diane Abbott, Britain’s first black female lawmaker, received almost half of all the abusive tweets sent to female members of Parliament.

Hillary Clinton, the 2016 U.S. Democratic presidential nominee, who was in London recently promoting her new book, told the BBC, “I’ve spoken to parliamentarians, current and former, in your country, who are genuinely afraid of all of the emotions and threats they see coming at them.”

Some of the lawmakers who stepped down in this election cited the abuse they receive as a contributing factor.

New research by Cardiff University and the University of Edinburgh found that a majority of those who voted for Brexit thought violence toward members of Parliament is a “price worth paying” for Brexit.

Richard Wyn Jones, co-director of the survey, said the results were “shocking and disappointing” and highlight “really staggering levels of polarization.”

He said that one factor that may be contributing to the “industrial quantities” of threats is that those on the losing side haven’t accepted defeat. And they haven’t accepted defeat, he said, because they feel they were “lied to, cheated and that the referendum was held under false pretenses.”

He added that on the winning side, “there was no attempt to reach out to the very, very large minority who voted a different way to say, ‘I hear your concerns, this is how we will assuage them.’ … Instead they are called ‘saboteurs’ or ‘remoaners’ or ‘traitors,’ and Brexit is redefined in an evermore hard-line way.”

Social media has amplified the problem, many say, with anonymity encouraging people to say things they otherwise wouldn’t. Some politicians have called for an end to anonymity; others are looking at new ways to encourage better manners online.

To be sure, politicians have long received threats and abuse, but many say it has become worse in recent years, especially for women.

Sam Smethers, chief executive of the Fawcett Society, an equal rights advocacy organization, said there has been an uptick in recent years of the number of female politicians prosecuting people for rape or death threats.

“It’s not just abuse and people not having a thick enough skin on social media. It’s actually a crossover between what people say online and what they are prepared to do offline,” she said.

Lawmaker Anna Soubry, who leads the Independent Group for Change and is well known for her views on Brexit, has received more abuse than most politicians. She recently received a letter at her office, written in all caps, that read: “COX WAS FIRST AND YOU’RE NEXT.”

One of the candidates running against her in the election was convicted of harassing her — she called her a “traitor” live on the BBC — and has been banned from visiting the constituency.

Soubry said that although she has received abuse since becoming a lawmaker in 2010, it grew worse after the Brexit referendum. She says what makes her “really cross” is that the incendiary language used by political leaders has encouraged others “to come out from the stone they clearly live under.”

Asked whether she has altered her behavior following the threats, she said, “I always keep a careful eye over my shoulder, wherever I am.”