Polls have closed in the Newark parliamentary by-election triggered by disgraced ex-Tory Patrick Mercer's decision to stand down.
The Conservatives are on course to hold off the threat from Ukip but only with a massively-reduced majority, according to polling.
Mr Mercer, who quit over what a sleaze watchdog said was the worst breach of transparency rules it had dealt with, won by 16,152 votes from Labour in 2010.
Ukip has put up MEP Roger Helmer - re-elected to the European Parliament in the party's "earthquake" victory last month - to take on Conservative Robert Jenrick in the constituency.
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In a tweet on Saturday, the US president didn't give a reason for his absence but said he wished"everyone well and have a great evening".