The outright condemnation from Yvette Cooper, the Shadow Home Secretary, and Labour backbencher David Lammy in the wake of Mr Farage's controversial comments about Romanians, came as polls suggested Ukip and the SNP were now vying for a Scottish Euro seat.
If the anti-Brussels party won, it would give Ukip its first elected representative in Scotland, and which, if the Nationalists won, would give them three out of Scotland's total of six MEPs.
There was also recognition from Liberal Democrat HQ last night that the party is bracing itself for a "difficult night" on Thursday as opinion polls suggest it could lose a considerable number of seats.
In a speech at Oxford University today, Nick Clegg will rage against the forces of insularity and chauvinism, saying: "Ukip. Conservative backbenchers. Isolationists. They are not thinking about Britain's interests. They shroud their narrow nationalism in the language of patriotism. They mask their hostility towards Europe as British bulldog spirit. But these are false patriots."
Ever since he suggested last week that there was a difference between how people should feel if Romanian or German people moved in next door, Mr Farage has faced a storm of criticism.
At the weekend, he sought to draw back from the comments, saying he had not used the "form of words" he would have liked and yesterday his party took out a full-page newspaper advert insisting it was not racist.
While David Cameron criticised the Ukip leader's "unpleasant" comments and Ed Miliband insisted they were "completely out of order", Ms Cooper went further, saying: "It's not racist to be worried about immigration or to want stronger controls but it is racist to somehow stir up fears about Romanians living next door. So Ukip should say they were wrong on that."
But Mr Lammy, who represents Tottenham in London, directed his criticism directly at Mr Farage. "What Nigel Farage said over the weekend was racist. So I'm clear; he's a racist," declared Mr Lammy.
In his party's advert, the Ukip leader said: "Let me be clear: Ukip is not a racist party and our immigration policy, far from being racist, aims to end discrimination against non-Europeans."
He admitted the "vast majority of Romanians who have come to the UK wish to better their lives and would make good neighbours" but Mr Farage stressed Britain should "not be in a political union with Romania with an opened door to all of their citizens".
Meantime, two snapshots put Ukip within touching distance of grabbing its first Scottish Euro seat. A Survation poll placed the SNP in front on 37%, followed by Labour on 26%, the Tories on 13% with Ukip on 10% and the Liberal Democrats and Greens both on 6%.
A separate ICM poll was broadly in line with these findings, putting the SNP on 36%, Labour on 27%, the Tories on 13%, Ukip on 9% and the LibDems and Greens on 7%.
Alex Salmond suggested voters had a clear choice between the Nationalist candidate Tasmina Ahmed-Sheikh and Ukip. "The intolerant message Ukip is peddling has no place in Scotland and the way to keep Ukip's agenda out of Scottish politics is by voting SNP," said the First Minister.
Scotland is currently represented by six MEPs: two SNP; two Labour; one Conservative and one Liberal Democrat.
l Last night, in the final ComRes poll for ITV News ahead of Thursday's poll, 33% of Britons certain to vote said they would opt for Ukip, ahead of Labour on 27%, the Conservatives on 20% and the Liberal Democrats on 7%. However, Ukip's lead appears to be narrowing. Over Labour, it is now only six points compared to the 11 in the previous poll at the end April; the party's vote share has fallen to 33%, down from 38%.