Defeat for the Conservatives in Thursday’s by-election in Brecon & Radnorshire would see the Government’s working majority in Parliament shrink to just one.
There are currently 320 MPs who – on paper – would back the Government in a crunch vote in the House of Commons – 310 Tories and 10 members of the DUP.
The DUP agreed to support the Conservatives in certain key votes, such as confidence motions and Budgets, as part of a deal reached in the aftermath of the 2017 general election.
Set against this combined total of 320 MPs are 318 MPs from all other parties.
The Liberal Democrats are favourites to win the seat, and, were they to do so, the opposition tally would increase from 318 to 319.
With the Tories and DUP unchanged on a combined total of 320, this would reduce the Government’s majority from two to one.
These totals do not include Sinn Fein’s seven MPs, who do not take their seats in the Commons, and the Speaker and three Deputy Speakers, who do not vote.
The Liberal Democrats need a swing of 9.8% to take the Welsh seat from the Conservatives.
It is the sort of swing which, were it to happen at a general election, could see the party pick up around a dozen seats from the Tories.
At the 2017 general election, the Conservatives’ majority was 8,038.
The party won 49% of the vote in 2017, compared with 29% for the Lib Dems, 18% for Labour, 3% for Plaid Cymru and 1% for Ukip.
In this week’s by-election, Plaid Cymru is not standing and has instead urged its supporters to vote for the Lib Dems.
The Green Party has done the same.
Both parties have stated that they want to try to maximise the chance of success for the Lib Dems.
The result of the by-election is expected to be declared at around 3am on Friday morning.
Here are the six candidates standing in the by-election:
Chris Davies (Conservative)
Tom Davies (Labour)
Jane Dodds (Liberal Democrats)
Des Parkinson (Brexit)
Liz Phillips (Ukip)
Lady Lily The Pink (Loony)