LONDON — As the shock of the brutal murder Thursday of a young member of Parliament began to subside, there was a growing sense in Britain that something dark had been unleashed in the country.

The increasingly ugly, anti-immigrant tone to the campaign over the referendum on Britain’s membership in the European Union, coupled with the violence of English fans at the European soccer championships, have left many feeling that the boundaries of acceptable behavior are breaking down.

“What we are just seeing generally is a very disturbing shift in British politics,’’ said Simon Tilford, deputy director of the Center for European Reform, which favors British membership.

With Thursday’s vote on the referendum only days away, campaigning was suspended as a gesture of mourning and respect for the victim, Jo Cox, 41, a rising star in the opposition Labor Party who was a strong backer of Britain’s remaining inside the bloc.

Although it is still too early to say how the attack will change the dynamics of the campaign, it has unquestionably shifted the focus from the growing momentum of those in favor of leaving to the anti-immigrant tactics they have employed as the vote has drawn closer.

The suspect arrested in the killing, Thomas Mair, 52, has a history of mental illness. But he was also reported to have been in contact with far-right groups in the United States and Britain, and to have said, “Britain first!’’ several times as he attacked Cox. Britain First, a far-right nationalist group, denied any links with Mair, but a US civil rights group said he had been associated with an American neo-Nazi organization called the National Alliance.

In a widely distributed piece written for the magazine The Spectator, which favors leaving the EU, Alex Massie drew a connection between the “Leave’’ campaign, which has featured outlandish assertions, xenophobia, and Islamophobia, to the death of Cox.

“Sometimes rhetoric has consequences,’’ Massie wrote. “If you spend days, weeks, months, years telling people they are under threat, that their country has been stolen from them, that they have been betrayed and sold down the river, that their birthright has been pilfered, that their problem is they’re too slow to realize any of this is happening, that their problem is they’re not sufficiently mad as hell, then at some point, in some place, something or someone is going to snap. And then something terrible is going to happen.’’

Massie cited a poster that had been issued earlier Thursday by one of the campaigns for British exit led by Nigel Farage, leader of the UK Independence Party, which has been strongly anti-Europe and anti-immigration from its inception and in May 2015 won 13 percent of the vote in the national election.

In the last stages, opponents say, the Leave campaign has essentially become a UKIP campaign on the putative dangers of immigration — given that membership in the bloc means freedom of travel and work for all its citizens — softened around the edges by Conservatives who say that as a great nation, Britain will be truly sovereign only away from Brussels.

The poster showed a long line of Middle Eastern refugees waiting to cross a European border. The text said: “BREAKING POINT. The E.U. has failed us all. We must break free from the E.U. and take control of our borders.’’ Never mind that the border in question was that between Croatia and Slovenia, and that Britain as an island nation is largely insulated from the immigration crisis that roiled the rest of Europe last year.

The message, Massie said, was not subtle. “Vote Leave, Britain, or be overrun by brown people. Take control. Take back our country. You know what I mean, don’t you: If you want a Turk — or a Syrian — for a neighbor, vote Remain.’’

Some observers, like Tilford, say there is blame to be spread across the political spectrum. Although the tenor of the Leave campaign “is pretty xenophobic,’’ he said, both of the major mainstream parties, Labor and the Conservatives, regularly discuss the potential menace of foreigners. “That has made it socially acceptable to voice anti-immigrant or xenophobic sentiment in a way that it wasn’t 10 years ago,’’ he said.

The more practical question is whether the murder of Cox will alter the trajectory of the campaign. Some analysts, like Mujtaba Rahman of the Eurasia Group, said it “will hurt the momentum of the ‘Leave’ campaign, which has been gaining steadily in recent polls.’’