British by-elections have a history of drama, and in the Welsh hills on Thursday voters will make a decision with lasting consequences for Brexit and Boris Johnson's new government.

What's happening?

Voters in Brecon and Radnorshire will choose a member of parliament in a special election, organized after the sitting Conservative MP, Chris Davies, was recalled and unseated in the wake of a conviction for cheating on his expenses.

The Welsh constituency, which voted 52-48 to leave the European Union in 2016, is a farming community – where sheep reportedly outnumber people by 10 to one – and the livestock sector is likely to be hit hard in the event of a no-deal Brexit. That growing possibility has already spooked currency markets this week. 

Who's standing?

Davies, the Conservative who was fined and ordered to complete 50 hours of community service for expenses fraud, is again on the ballot and will be challenged from the right by the hardliner Brexit Party.

Crucially, two Remain parties – the Greens and the Welsh nationalist Plaid Cymru – have agreed to stand aside to support the pro-EU Liberal Democrat contender Jane Dodds. This is a pattern likely to be repeated if a general election is held in the coming months.

In the 2015 election, Davies won 5,000 more votes than his incumbent Liberal Democrat rival and extended that lead to 8,000 in 2017. The Labour Party came a close third in 2017, but its mixed messages on Brexit are again expected to hurt in the by-election.

Why does it matter?

Ultimately, the by-election matters because, like everything else in British politics at present, it has a bearing on Brexit.

The new government headed by Johnson has a majority of just two MPs and the Brecon poll is its first electoral test, with a general election an increasingly likely prospect in the near future. 

The Conservatives have 310 voting MPs and are supported by 10 Democratic Unionists ahead of the by-election. The opposition parties have a combined total of 318 voting MPs.

If the Liberal Democrats win the seat from the Conservatives, the governing majority will fall to one, another opponent to a no-deal exit from the European Union will take a seat in parliament and just one defection would be needed for Johnson's government to lose a no-confidence vote. It would also be a fillip for the Liberal Democrat's new leader Jo Swinson, who is strongly in favor of the UK staying in the EU.

The Conservative decision to run a recalled candidate, together with pressure from the Brexit Party, is likely to dampen its support, but Davies did hold a large majority in 2017. If he can pull off a victory it will be a major boost for Johnson and help him maintain early momentum; a loss would burst the Boris balloon and weaken his chances of delivering Brexit by October 31.