The hidden bonus for the Lib Dems in striking an alliance with Plaid Cymru and the Green Party

Don’t mention the coalition. 

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Winning here? Plaid Cymru has agreed to stand aside in the Brecon and Radnorshire by-election. The party is urging its supporters to back the Liberal Democrat candidate Jane Dodds, while the local Green Party has also opted not to stand. 

As far as the electoral dividend to Dodds is concerned, it isn't much to get excited about: neither Plaid Cymru nor the Greens have any history of significant success or even a particularly large vote to squeeze in the area. But is it the beginning of a wider pattern of cross-party cooperation, as Adam Price, Plaid Cymru's leader, hopes? 

As far as the Liberal Democrats and Greens are concerned, the two parties' success in the local elections was in part down to a series of cross-party pacts between local parties, alliances that Vince Cable and Caroline Lucas paved the way for with deals in their own constituencies in the 2017 election. 

There is a much stronger streak of independent local government representation in Welsh local elections, which makes it harder for local parties to build trust and close links to pull these agreements off. 

But it is also easier for Plaid Cymru to reach an accord and to sound open to the idea because it already holds Ceredigion, the only real Liberal Democrat-Plaid Cymru battle in the country, so signalling to Welsh Remainers that you should vote for whichever pro-Remain party is stronger locally is win-win for Plaid Cymru, particularly as it boosts their hopes of taking seats from Labour if it can further squeeze the Liberal vote.

It's harder to see what the Liberal Democrats get from it, which is why the party is divided over the issue. The anxiety among some Liberal Democrats about these pacts is that they are giving away prime targets in exchange for not much: look, they say, at the Green Party, who happily worked with us in the local elections and then blasted us for our time in the coalition at the European elections. What's the use of the Lib Dems picking up support in Brecon and Radnorshire, which they ought to win unaided, if it blocks their path in Ceredigion? 

But while the direct electoral upsides for the Liberal Democrats are unclear, there is a big political prize on offer. One problem that party has, or at least used to have until very recently – was that their participation in coalition was a deal-breaker for some voters.

The attack line on them from Labour and the Greens at the next election will be all about the coalition, but as the Green Party found in the European elections, it's hard to successfully work with someone at the start of the month and convince voters that they should never be forgiven at the end. And that the other parties of the pro-European centre and left are now frequently and openly signalling that the Liberal Democrats are no longer persona non grata is a bigger prize to that party that any one parliamentary seat. 

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.