A new poll shows 26 per cent would vote for a Green candidate capable of winning, compared to 24 per cent for Ukip.
Polls have now opened in the constituency where immigration dominates debate, and Ukip is poised to seize power from the Tories.
Losses to Ukip mean the party has failed to gain any ground since 2010.
A fundamental economic tension runs through Ukip.
The Ukip leader on coalitions, immigration fears and why he’s chasing the Labour vote.
The Tory-turned-Ukip defector talks by-election tactics, potential future defections, and how he'd prefer sitting with Ukip MPs in Westminster than Holocaust-deniers in the European Parliament.
Farage's party badly defeated after attempting to exploit Rotherham child abuse scandal.
Absent-minded protagonists? The UK parties are contributing to a political crisis in Northern Ireland.
Tory turncoat Douglas Carswell has opted to sit in the Commons chamber on the front bench below the gangway populated by the socialist heckler tendency.
Trailing to Ukip on immigration and to Labour on the NHS, the Conservatives' best hope may be an economic slowdown that persuades voters the storm has not passed.