The Lib Dems think they will snatch a narrow win in Richmond Park tomorrow and dethrone the former Tory MP Zac Goldsmith.
Polls open in South West London tomorrow, and Goldsmith had been regarded as favourite, but bookmakers are also cutting the odds on victory for Tim Farron's party.
And internal modelling by the Lib Dems is now predicting a knife-edge victory for their candidate, former accountant Sarah Olney.
Lib Dem figures are predicting a 47.2 per cent vote for Olney, compared to 45.8 per cent for Goldsmith.
It marks a swing from last week, when the party still believed Goldsmith to be in the lead.
In part the movement is attributed to growing support from Labour voters, who Lib Dems suspect are moving to back Olney over former London mayoralty candidate Christian Wolmar.
Olney has repeatedly sought to focus the by-election on Brexit, vowing to fight any move to quit the EU, while the Goldsmith camp has been hoping his strong personal brand, burnished by his resignation on Heathrow, will see him through the contest.
A Lib Dem victory would mark a dramatic shift from last year's general election result, which saw Goldsmith win through with 60 per cent of the vote in the constituency.
But it would also indicate growing momentum for the party. Although they remain far behind in national polling, the Lib Dems also secured a vastly improved result in a by-election in the seat for former Prime Minister David Cameron.
Richmond Park by-election candidates:
- Zac Goldsmith, Independent
- Howling Laud Hope, The Official Monster Raving Loony Party
- Maharaja Jammu and Kashmir, One Love Party
- Sarah Olney, Liberal Democrats
- David Powell
- Dominic Francis Stockford, Christian Peoples Alliance
- Fiona Natasha Syms, Independent
- Christian Wolmar, Labour Party