A large UK Independence Party (UKIP) poster is displayed in the campaign offices of Douglas Carswell in Clacton-on-Sea, England Tuesday. By touting its populist message, the U.K. Independence Party could tip the balance in next year’s election. (Matt Dunham/AP)

The 70-mile stretch of sea that separates this faded Victorian resort town from mainland Europe has long served as a bridge linking the British islands to the continent.

But on Thursday, voters here are expected to turn out en masse to back a party that wants to turn the bridge into a moat.

The U.K. Independence Party, which has enjoyed a surge in popularity this year with calls for Britain to get out of the European Union, is considered the overwhelming favorite to win its first-ever parliamentary seat in a special election here in Clacton-on-Sea.

The victory would mark another milestone in the party’s journey from the xenophobic fringe to the center of British political discourse following its triumph in European parliamentary elections in May — the first time in more than a century that a party other than Labor or Conservative has won a nationwide British vote.

While one seat in the 650-member House of Commons will hardly make UKIP a force in the Palace of Westminster, the situation here in Clacton reflects a broader political dynamic that could spell major trouble for Prime Minister David Cameron as he seeks another five-year term next May.

Douglas Carswell, a member of the British parliament who recently defected from the Conservative Party for the U.K. Independence Party, says British government is in need of great change. (Griff Witte/The Washington Post)

“It’s possible — and some would say likely — that UKIP could cost David Cameron and the Conservatives the next general election,” said Matthew Goodwin, a politics professor at the University of Nottingham and author of a book chronicling UKIP’s rise.

That’s because UKIP appears poised to siphonvotes from the right and deny the Conservatives seats they would otherwise win. Even if UKIP gains only a handful of seats, its share of the vote could tip close contests to the center-left Labor Party and help deliver the prime ministership to Labor leader Ed Miliband — despite the fact that a significant majority of voters consider him unlikable and ill-prepared for the job.

Cameron is all too aware of the threat and has sought to warn Brits away from Nigel Farage, the beer-swilling former commodities trader who has, through force of personality, transformed UKIP into a populist phenomenon.

“On 7 May you could go to bed with Nigel Farage and wake up with Ed Miliband,” Cameron told the Tory faithful at the annual party conference last week.

But that warning appears to have little resonance here in Clacton, where opinion polls show the UKIP candidate with a nearly insurmountable lead heading into Thursday’s vote.

Adding to the sting for the Conservatives is the fact that the UKIP candidate was, until just over a month ago, a Tory.

Douglas Carswell has represented this coastal region in parliament for the past decade. But he defected from the Conservatives at the end of August, saying he had lost faith in Cameron’s ability to deliver on his promises — including a pledge to give British voters a referendum on European Union membership by 2017.

Nigel Farage, left, leader of the UK Independence Party, and John Bickley, the UK Independence Party candidate for the Heywood and Middleton constituency, order a pint of beer in The Gardeners Arms pub after unveiling a campaign poster in Heywood, north-west England Tuesday. (Oli Scarff/AFP/Getty Images)

Now Carswell, a 43-year-old former fund manager who has written wonkish books about how the Internet can transform democracy, is running as a renegade outsider.

“It’s time to break open the corporatist racket called politics,” the fit and energetic Carswell said this week during a break between campaign stops.

Asked about Cameron’s appeal to voters to make a pragmatic choice in order to keep Labor out of power, Carswell described it as “patrician, aloof and arrogant.”

“It typifies everything that’s wrong with our politics,” he said. “The premise of that argument is that certain voters belong to certain people. It’s ridiculous.”

Carswell’s anti-establishment message captures the British mood well. Just last month, 45 percent of Scots voted to leave the United Kingdom, a result as much about voters’ contempt for the political establishment in London as the appeal of Scottish nationalism.

The coastal swath of southeast England that Carswell represents is especially contemptuous of the country’s political elite. Despite overall gains in British GDP, the economy here has struggled, with stagnant wages, rising prices and an older, less-educated population that lacks the skills for higher-paying jobs.

“These are people who feel that modern Britain has left them out,” said Peter Kellner, president of the polling firm YouGov.

For many such voters, the problem isn’t just London politicians. It’s also the open-borders policy of the European Union, which has allowed millions of people to flood into Britain over the past decade — including vast numbers from the continent’s struggling south and east.

Helen Morter, a 60-year-old retired hairdresser, was volunteering in the bustling UKIP campaign office this week despite being a stalwart Tory “since I was knee-high to a grasshopper.”

She was inspired to switch, she said, by Carswell’s defection — but also by anger at the immigrants she says are changing the character of this seaside community.

“Go on one of the rides at the pier. They’re all Polish down there. And they’re taking our English jobs,” Morter said. “There are just too many foreigners here. And if that makes me sound like a fascist, well, then maybe I am.”

Such comments help explain why UKIP has faced persistent accusations that its anti-E.U. stance is just a cover for anti-foreigner prejudice.

Carswell has tried to broaden the party’s appeal, focusing his campaign on local concerns — streetlights that go out at midnight — and on national issues such as energy, health care and education that are not normally associated with UKIP.

Immigration barely appears in his campaign literature, and he discusses the issue only when others bring it up.

“We need to make sure that we don’t blame outsiders for the failures of our political insiders,” he said.

Still, the desire to blame outsiders is strong here — even among voters who say they will shun UKIP and stick with the Conservatives. That leaves Cameron in the delicate position of having to vie with Miliband for centrist voters while also appeasing an increasingly vocal right wing that wants the country to turn inwards.

Walking along the town’s ragged seafront, across from a shuttered amusement park, 79-year-old Jack Carr said he will stay loyal to the Tories but can’t wait to see Britain leave the burdens of Europe behind.

“We’re lifting everyone else out of the gutter,” said Carr, a retired shoe repairman, “and putting ourselves right back in it.”