(Reuters) - The anti-EU UK Independence Party looked poised to win its first ever elected seat in parliament on Thursday, dealing a symbolically ominous blow to Prime Minister David Cameron seven months before a national election.

Such a breakthrough for UKIP could demonstrate an ability to split the mainstream Conservatives' vote and thereby cloud their re-election prospects. It would also raise pressure on Cameron to become more Eurosceptic three years before a referendum on European Union membership to be held if he is re-elected.

Opinion polls suggest UKIP, which favors a British EU exit and lower immigration, will easily win a special election in Clacton-on-Sea, southeast England, where voting gets under way at 0700 London time and results are expected on Friday around 0200 London time.

"British politics will never be the same again," Patrick O'Flynn, a UKIP lawmaker in the European Parliament, forecast on Wednesday. "And amen to that."

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 MPs are skeptical about his resolve to push for real change, viewing his promise as a tactical move to try to hold his divided party together.

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 told Reuters.

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 Labour lawmaker, a result it says 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 MPs in the last six weeks, and polling suggests it may win up to six of 650 seats in the British parliament next year.

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 Labour party, strikes fear into Conservative hearts.

Deputy Prime Minister Nick Clegg, leader of the Lib Dems, the junior partner in Cameron's coalition, said on Wednesday that UKIP was winning voters over by appealing to their fears and by providing them with scapegoats.

"Life is so simple when you know who or what to blame," he told his party conference in Glasgow.

"It's seductive and it's beguiling. That much may even be proved tomorrow, if the people of Clacton give the UK Independence Party an MP. But resentment, the politics of fear, doesn’t pay the bills or create a single job."

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 Labour - have taken turns to govern.

But although a blow to Cameron, Conservative strategists say, a defeat in Clacton would be manageable. They are more worried about the possibility of defeat in a second special election because it appears more winnable for them.

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.