Voters go to the polls on Thursday to vote in the Brecon and Radnorshire by-election. It will be the first electoral test of the new prime minister, Boris Johnson.
But why does this particular election matter so much?
Given the government's wafer-thin majority in the House of Commons, there is a lot at stake.
The Conservatives are defending a majority in the constituency of just over 8,000. Should they lose the seat, it will cut their working majority in Parliament to just one.
Such an outcome could make it harder for Boris Johnson to deliver Brexit on 31 October and would leave the prime minister more susceptible to losing a possible motion of no confidence.
The anti-Brexit Liberal Democrats will be seeking to capitalise. Their new leader, Jo Swinson, will be keen to make an immediate impact. Her party previously held Brecon, before it fell to the Conservatives in 2015.
The Lib Dems have been supported by two other anti-Brexit parties - Plaid Cymru and the Greens. Both have agreed not to stand this time, to give the Lib Dems a better chance of winning.
The candidates standing in the election are: Chris Davies, Conservative Party; Tom Davies, Labour Party; Jane Dodds, Liberal Democrats; Des Parkinson, Brexit Party; Liz Phillips, UKIP; Lady Lily The Pink, Monster Raving Loony Party.