Remain parties are working together for a by-election, taking place days after the new prime minister comes to power.
The Liberal Democrats and Plaid Cymru are joining forces in the hope of winning the Brecon and Radnorshire by-election next month.
Plaid Cymru are not putting up their own candidate, but have instead asked supporters to vote for Lib Dem candidate Jane Dodds instead.
The Green Party has also decided independently that it won’t stand.
The party said it made the decision “in order to maximise the chances of the candidate most likely to beat the Tories and the Brexit Party”.
The by-election was triggered after the Conservative MP for the seat, Chris Davies, was convicted over a false expenses claim and then unseated by a recall petition, where 19% of the constituency’s voters said there should be a by-election.
The Tories have re-chosen Mr Davies to stand for them in the contest on Thursday 1 August.
The Liberal Democrats finished second in the constituency at the last general election, and held the seat from 1997 until 2015.
Lib Dem leader Sir Vince Cable told the BBC the partnership between the two parties was “a very welcome development”, and it made sense that “the two strongest Remain parties in Wales are going to work together”.
Adam Price, the leader of Plaid Cymru, said it was a “major step”, but it was “the right thing to do”.
He added: “We are facing one of the most significant decisions, as to whether we are going to be seemingly yanked out of the European Union even without a deal.
“Under those circumstances it is in Wales’s interest and our common interest to work together and coalesce the support for the Remain side in Wales.”
Both leaders hinted this could be the start of further co-operation between the parties, but neither were willing to be specific.
“There is no doubt that the co-operation that this is generating could well lead to wider measures,” Sir Vince said.
Mr Price added: “This is a first step. We have committed to continuing to explore how we can work together in future elections as well.”
In the 2017 general election, Tory Chris Davies won the seat with a majority of 8,038, with the Lib Dems in second place and Plaid Cymru fourth.
In the European Parliament elections in May, the Brexit Party won two of Wales’s four seats, with Labour and Plaid taking one each.
And in the EU referendum, 52.5% of participants in Wales backed Brexit.
For this by-election, Tom Davies will stand for Labour in the by election and Des Parkinson is the Brexit Party’s candidate.