Brecon showed what can happen when Remain parties unite. Could the tactic see Boris beaten at a general election?

The Remain movement is alive and kicking; reports of its death after Boris Johnson’s arrival in Downing Street were greatly exaggerated. The Liberal Democrats owed their victory in Thursday’s by-election in Brecon and Radnorshire to a Remain alliance in which the Greens and Plaid Cymru stood aside.

The result should give Johnson pause for thought about an early general election, at least before he has “delivered Brexit”. The Lib Dems have momentum under their new leader Jo Swinson and on present form, could increase their 13 seats close to their high watermark of 62 MPs in 2005.

Brecon showing the Lib Dems can win in areas where they have traditionally been strong is ominous for the Tories, who won an overall majority in 2015 by seizing Lib Dem strongholds in the southwest. Boris’s drive towards the no-deal cliff risks writing off the estimated 4 million 2016 Remainers who backed the Tories in 2017.

He will need to pick up Labour Leave voters to compensate. This is why he love-bombs the Midlands and north with money.

Johnson might need an electoral pact with Nigel Farage – dubbed a “regressive alliance” by campaigners for a “progressive” one. Boris has dismissed the idea, but might have to think again; if the 3,331 voters who backed the Brexit Party in Brecon had supported the Tories, they would have won the by-election by 1,906 votes (more than the Lib Dems’ 1,425 majority).

Farage has sent conflicting signals about a pact. He will probably play hard to get. He has been scathing about Dominic Cummings, Johnson’s senior adviser, claiming he is not a “true believer”.

The prime minister’s problem is that, while he would prefer an election “after Brexit”, he might well have to fight one before he can deliver it – either seeking one because he knows the Commons will block no deal or being defeated in a no-confidence vote.

In such an election, could a Remain alliance thwart Johnson’s hopes and scupper Brexit? The test run in Brecon certainly provides grounds for optimism. There are other hopeful signs. James Kanagasooriam, a political analyst, has identified 66 seats in England and Wales where such an alliance could potentially achieve victory. Two-thirds are Tory-held.

However, a Remain alliance could also draw votes away from Labour, allowing the Tories to win some of the Con-Lab marginals that usually decide elections. Privately, Tory strategists hope this would happen.

Under a different leader, Labour would be heading a Remain alliance. But Jeremy Corbyn is a tribal figure. Even when he finally pledged a referendum on any Tory Brexit and to support Remain in it, Labour insisted it is not “a Remain party”. Its general election stance remains a mystery to itself, let alone the voters.

Some anti-Brexit campaigners would love to forge a nationwide alliance. That is not going to happen. Heidi Allen, the former Tory and Change UK MP who is now an independent, has launched Unite to Remain, which she hopes will result in a single Remain candidate in 150-180 seats. That will probably prove optimistic. However vital the national interest on Brexit, old local rivalries die hard.

Candidates in place are often reluctant to stand aside. When the Liberal Party attempted a national carve-up of seats with the Social Democratic Party before their eventual merger, it was a nightmare. Top-down does not work. National leaders should encourage pacts, but they are best sorted locally. Even if that limited the number of deals to around 50, it could yield a big dividend in a four-party race likely to result in another hung parliament.

Although a Remain alliance could pay dividends in Wales, it would not work in Scotland. The Scottish National Party, while happy to cooperate with other parties at Westminster, is not going to help the Lib Dem revival spread north of the border. The SNP has its sights on Swinson’s East Dunbartonshire seat, which it held from 2015-17.

Swinson will have a big say in whether a Remain alliance achieves lift-off. The Greens and Plaid Cymru will rightly demand some payback for the Lib Dems’ free run in Brecon. The Greens will be wary. Their fingers were burnt in 2017, when they were more open to a progressive alliance but their actions in standing aside were not reciprocated.

Some Lib Dems will be cautious about a nationwide pact, arguing that their revitalised party is the biggest kid on the Remain block and so should get the lion’s share of any seats. Fortunately, Swinson is more open to cross-party cooperation than some colleagues. Her instinct will be to be a generous partner in a Remain alliance, prepared to make some sacrifices for the bigger cause. She should do so, and the other Remain parties should respond in kind. It could make all the difference when the election comes.