Votes are being counted in the by-election triggered by the resignation of Conservative MP Stephen Phillips over ''irreconcilable policy differences'' with the Government.
Ten candidates are contesting the Sleaford and North Hykeham seat where the high-flying barrister had a majority of more than 24,000 at last year's general election.
The former MP supported Brexit but criticised ministers' handling of the issue in Parliament, as well as their approach to child refugees and targeting of foreign aid.
Caroline Johnson, a consultant paediatrician, is hoping to defend the long-standing Tory seat.
Ukip is set to pose the biggest challenge to her bid.
Former Ukip leader Nigel Farage visited the seat to canvass support for the party's candidate, Victoria Ayling, and suggested a purple upset in the seat was not impossible.
''I don't know, it's 2016, why predict anything?" he said during the visit. ''Only an idiot predicts things in 2016 because it's been so full of upsets."
Mr Farage's successor Paul Nuttall also visited the Lincolnshire seat during the campaign in a sign of Ukip's ambitions.
Labour candidate Jim Clarke is a GMB union member and refuse collection driver, while the Liberal Democrats are fielding Ross Pepper, who works for an optician.
The Green Party is not fielding a candidate but has given its backing to an independent, Sarah Stock, who is campaigning on a ''Save the NHS'' ticket.
Prime Minister Theresa May will be hoping voters do not deliver the upset they returned in the Richmond Park by-election a week ago.
That vote was sparked by the resignation of Zac Goldsmith in protest at the Government's decision to back a third runway at Heathrow.
Although he stood as an independent, the Conservatives did not field a candidate in an ultimately failed attempt to stop the Liberal Democrats from taking back the once yellow seat.