With polls showing the UK Independence Party will come second in the May 22nd election, ahead of Prime Minister David Cameron's Conservatives but behind the opposition Labour Party, UKIP hopes to attract enough extra votes to win the ballot outright.
Party leader Nigel Farage has said such a victory would trigger a "political earthquake", putting UKIP in a position to win its first seats in the British parliament in 2015 and piling pressure on the three main parties to sever Britain's ties with the European Union.
UKIP wants Britain to leave the EU and an end to "open door" immigration, particularly from inside the 28-nation bloc. Polls show immigration to be one of voters' top three concerns and Cameron is under pressure to make good on his promise to cut net migration to the "tens of thousands" by 2015.
The prime minister has also promised Britons a referendum on leaving the bloc if re-elected next year. Labour and the Liberal Democrats oppose such a vote.
Launching its campaign in the northern English city of Sheffield, UKIP released a series of posters urging voters to "take back control of our country" by electing UKIP candidates. The party currently holds nine seats in the European Parliament.
One poster featured a picture of a finger pointing to anyone reading it, saying "26 million people in Europe are looking for work. And whose jobs are they after?"
Another showed a picture of a man dressed as a construction worker begging on the street, saying: "EU policy at work. British workers are hit hard by unlimited cheap labour".
Mike Gapes, a Labour lawmaker, said the posters were "racist", while Nicholas Soames, a Conservative lawmaker and the grandson of Winston Churchill, condemned them as "deeply divisive, offensive and ignorant."
"This UKIP campaign is a racist, xenophobic campaign designed to win votes by whipping up animosity against foreigners living and working and contributing to this country," Gapes wrote on his web site.
Farage, who is married to a German citizen, rejected the accusations, saying the posters were aimed at highlighting the fact that an influx of cheap foreign labour had driven down wages and increased youth unemployment.
"This is the classic trick," he said. "Any debate on immigration is closed down automatically. Decry it as racism. Don't talk about it. Brush it under the carpet. We're not going to do that."
UKIP was a non-sectarian, non-racist political party, he said, and the posters were a "hard-hitting reflection of reality".
Farage, a member of the European Parliament, was later put on the spot over his German wife being his secretary when a reporter asked him how he could justify her employment while complaining that foreigners were taking British jobs.
"It's a very different situation to a mass of hundreds of thousands of people coming in and flooding the lower end of the labour market," said Farage, saying she was in a unique position as his wife to work extremely unsociable hours.