By Andrew Osborn
CLACTON-ON-SEA England (Reuters) - Britain's anti-EU UK Independence Party was poised to win its first elected seat in parliament on Thursday, highlighting the threat it poses to Prime Minister David Cameron seven months before a national election.
Such a breakthrough would demonstrate UKIP's ability to split the mainstream Conservative party's vote and cloud its re-election prospects in 2015.
It would also raise pressure on Cameron to become more Eurosceptic, three years before a referendum on European Union membership which he has promised to hold if he is re-elected.
Opinion polls suggest UKIP, which wants Britain out of the EU and lower immigration, will easily win an election in Clacton-on-Sea, southeast England, where voting got underway at 0600 GMT (2.00 a.m. EDT). Results are expected on Friday around 0100 GMT.
"It's in the bag," predicted UKIP supporter Annie-Therese Murray, walking through Clacton with a sandwich board urging people to vote for her party.
"This success will be repeated elsewhere," she said, saying she had traveled from Scotland to help out with campaigning.
UKIP's candidate in Clacton, Douglas Carswell, an arch Eurosceptic, defected from Cameron's Conservatives in August, triggering Thursday's vote. He said at the time that he had switched allegiance because he doubted the prime minister's determination to reform the EU.
Cameron has promised to try to renegotiate Britain's EU relationship before offering voters an in/out membership referendum in 2017. But many of his own lawmakers are sceptical about his resolve to push for real change, viewing his promise as a tactical move to try to hold his divided party together.
With a population of 53,000, Clacton, once a thriving seaside resort, lost out to Britons' love affair with cheap foreign package holidays and now earns its keep from day trippers from London and retirees.
Retirement homes line the seafront, gaudy arcades filled with slot machines and bookmakers dominate the town center, and caravan parks luring low-income families with cheap deals sit on the outskirts along with Jaywick, an area officially rated as one of the most deprived in the country.
Reading newspapers in his souvenir shop at the end of Clacton's 19th-century pier, David Ashton, 66, said he had voted UKIP because he had lost faith in Cameron.
"Ever since I was old enough to vote I have always voted Conservative," he told Reuters. "But this time I voted UKIP. This place needs a shake-up. The Conservatives have promised stuff before and not delivered. I don't trust them any more."
He cited their failure to curb immigration and what he said was their desire to remain in the EU, something he opposed.
In terms of its demographics, Clacton is the most UKIP-friendly constituency in the country, according to analysis by academics Matthew Goodwin and Robert Ford.
"It's very white, very old, very working class, lots of economic deprivation and ... there is a heightened anxiety over migration and Europe," Goodwin said.
UKIP is also expected to poll strongly although not win in another special election being held on Thursday in northern England after the death of the area's opposition Labor member of parliament. UKIP says that result will show it is a threat to the established left as well as the right.
Tapping into a weariness with mainstream politics, UKIP won European elections in Britain in May, poached two of Cameron's lawmakers in the last six weeks, and polling suggests it may win up to six of 650 seats in the British parliament in next year's national election.
That sounds like a paltry number. But its ability to split the centre-right vote in any number of constituencies which, under a winner-takes-all system could hand them to the Labor party, strikes fear into Conservative hearts.
Cameron's strategy to stem UKIP's rise has been to warn voters that "a vote for UKIP is a vote for Labor".
Cameron, who once derided UKIP as a bunch of "fruitcakes, loonies and closet racists", has also said his is the only party able to deliver a referendum on EU membership.
UKIP believes its success will begin to unravel a political establishment under which until recently Britain's two main parties, the right-leaning Conservatives and left-leaning Labor, have taken turns to govern.
"It's the beginning of change," Margot Parker, a UKIP lawmaker in the European Parliament, said in Clacton.
"It will be a blow for Cameron but the Conservative party is not the party it was 20 years ago. They left their own voters behind when they moved in a more liberal direction without listening to the grassroots."
But though a setback to Cameron, Conservative strategists say a defeat in Clacton would be manageable. They are more concerned about the possibility of defeat in another special election.
Triggered by another defection to UKIP, the election expected in November will be in Rochester, a part of southern England where voters are seen as less UKIP-friendly and where the UKIP candidate Mark Reckless, a former Conservative, is regarded as far more vulnerable than Carswell.
(Additional reporting by Kylie Maclellan; Editing by Janet Lawrence)