Polling closes at 22:00 GMT tonight in three by-elections, one of which could conceivably return a Ukip MP.
Voting is being held in Croydon North, following the death of MP Malcolm Wicks; Middlesbrough, following the death of MP Stuart Bell; and Rotherham, following the resignation of Denis MacShane.
All are safe Labour seats, but Ukip is hoping that the swirl of publicity surrounding the party over the last week will allow it pull off a shocking win in Rotherham.
Labour enjoys a majority of 10,462 - 71% of the vote - in the constituency, which the party has controlled since the early 1990s.
But pollsters believe voters are angered by MacShane's expenses scandal and are attracted by Ukip's hardline policies on immigration.
The party's support was bolstered by the outrage over Labour-run Rotherham council's decision to take three children from their foster parents because the couple were members of Ukip.
Tory vice-chairman Michael Fabricant then offered the party more free publicity when he suggested a Conservative-Ukip pact.
The far-left Respect party is also contesting the seat, hoping for a shock result along the lines of George Galloway in Bradford West. It has poured resources into the campaign and deployed Yvonne Ridley, a former journalist who converted to Islam after being held captive by the Taliban.