Britain's anti-European UK Independence Party (UKIP) was on Thursday set to claim its second seat in parliament a month after gaining its first foothold as voters went to the polls in Rochester.
The by-election in southeast England was called after MP Mark Reckless defected in September from Prime Minister David Cameron's Conservative Party to UKIP, which wants strict quotas on immigration.
The Conservatives have campaigned hard for the Rochester and Strood seat, leading experts to portray this as a historic moment in British politics if polls indicating a UKIP win bear out.
"UKIP was not supposed to win this by-election," explained Matthew Goodwin, politics professor at Nottingham University.
UKIP victory in Clacton on October 9 was more predictable as it was "perfect territory for UKIP".
"It is filled with the types of voters who have fuelled UKIP's rise since 2010 - older, white, working-class and struggling voters who have few qualifications.
"But Rochester and Strood is a different matter - which is why the Tories were confident they would smash UKIP. It is way down the rankings in the UKIP-friendly table at number 271," he added.
Cameron vowed to "throw everything" at the battle and defeat would deal a blow to his reputation, but that could turn into a full-blown crisis if the result triggers further defections.
Reckless on Wednesday said that two more Tory MPs were thinking of jumping ship, increasing pressure on Cameron ahead of next year's general election.
The prime minister has already promised a referendum on Britain's EU membership if his party wins the general election and has taken a harder stance on immigration in an attempt to stem the flow.
Eric Pickles, the Conservative communities secretary, dismissed Reckless's claims.
"I don't think there will be any (defections)," he said during a campaign trip to Rochester.
"We have brought the economy back from the brink. At times when we talk to our friends in UKIP it sounds as though the only thing they really like about our country is its past."
Polls opened at 7:00 am local time (0700 GMT) and will close at 10:00 pm with results expected early Friday.
As campaigning entered the final straight, UKIP was on Wednesday forced to clarify remarks made by Reckless over its immigration policy.
The party insisted it was in favour of allowing all existing EU migrants to stay, despite Reckless appearing to suggest that they would only be allowed to remain in the UK for "a transitional period" if Britain left the bloc.
Despite the polls, Conservative candidate Kelly Tollhurst maintained that she could pull off an unlikely victory, and called upon voters of all stripes to lend her their support.
"It's a two-horse race and the reality of it is if we don't want to wake up to a UKIP MP on Friday then people who would normally be voting for Labour, the Lib Dems and or anyone else, need to be voting for me," she said.