>>730 That's an interesting question. The assumption before the election was that Farage would take voters from the right - however it now appears that more damage was done to the left (http://www.theguardian.com/commentisfree/2015/may/23/labour-win-working-class-voters-ukip).
The theory is that white working class men from the North, traditional Labour supporters, are most likely to feel threatened by immigration because immigrants are competing for the same unskilled work and are often prepared to work for lower wages. They are therefore likely to respond to the UKIP message of no immigration and to be turned off by Labour's silence on this issue. Labour has tended to accuse the Tories of racism for wanting to debate immigration, but that position has tended to mean Labour hasn't addressed the issue at all, or done enough to address the concerns of those who feel that "jobs are being stolen.