Irish actor Owen McDonnell could not sleep one night last month. He was due to start shooting on a new film the next day and made the mistake of reading comments by a Brexiteer in a news report. His anger stirred him into action.

“In the middle of the night I decided to join the Liberal Democrats because I was sick of not doing anything and because this is a potential seat to win. I said: ‘Screw this – I have got to join up and make a difference,’ ” he said.

A resident of St Albans, a commuter town north of London that is home to City lawyers and bankers, the Galway actor – better known for his roles in the RTÉ drama Single-Handed and the international BBC hit Killing Eve – sees the Lib Dems as “the party of moderation” in the bitter fight over Brexit that has left the Conservative and Labour parties “disunited” and pushed to extremes.

Describing himself as a “natural Labour voter”, McDonnell, who has been living in the UK since the mid-1990s, voted for Diane Abbott when he lived in London until he moved here last year. Unlike her safe-as-houses Labour seat, St Albans is considered up for grabs in Brexit-contaminated British politics when voters are expected to head to the polls in the coming weeks or months.

Daisy Cooper, the Lib Dem candidate in the constituency, increased her party’s vote by 14 per cent in the 2017 election and reduced the Tory majority by half. She is eyeing the seat held since 2005 by pro-Brexit Conservative MP Anne Main in a constituency that voted 63 per cent Remain in the 2016 Brexit referendum.

The Lib Dems, who under leader Jo Swinson are pushing for a second referendum or “People’s Vote” with the option to remain in the EU, are poised to campaign in the next election to revoke the article 50 exit process.

McDonnell, like his preferred candidate in St Albans, spies an opportunity. He believes the party offers a “sensible approach” to Brexit that recognises that “half the country didn’t want it to happen”.

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‘Balance of power’

“The Liberal Democrats are in a position where they can actually say, ‘look, we could hold the balance of power and if we do, we are in a position to temper either the Conservatives who have become increasingly right-wing under Boris Johnson or Labour who become increasingly left-wing under Jeremy Corbyn,” said McDonnell, sipping a coffee near his home.

“So I think voting for the Liberal Democrats is not a protest vote; it is a vote that can actually make a meaningful difference to the way the country is going.”

The actor is not alone.

In St Albans Baked Nation coffee shop, Cooper told The Irish Times that the desire to unseat a pro-Brexit in a Remain constituency is drawing new supporters who have not traditionally voted for her party.

Liberal Democrats candidate in St Albans Daisy Cooper: “Prorogation has created a real visceral anger on the doorsteps.” Photograph: Simon Carswell

“We are attracting a lot of soft Labour voters who would tactically vote for us and that has been the strategy for us for a while, but what’s quite surprising now is that we are winning a lot of soft Conservatives as well,” said Cooper, who describes herself as “a committed Remainer” and campaigner for a second Brexit vote for three years.

Of almost 1,000 new members to have joined the party this year, she says they are evenly split among former Conservative voters, former Labour voters and “lifelong liberal voters who haven’t felt mobilised before”.

Party allegiances

Less than half of the Labour candidate’s votes in 2017 would have been enough for Cooper to take the Tory seat.

St Albans is one of dozens of constituencies across the UK targeted by the People’s Vote campaign for tactical voting where the Remainers are being urged to set aside party allegiances in favour of pro-Remain candidates with the best chance of winning to shore up enough support at Westminster to block Brexit.

Cooper says Boris Johnson’s decision to prorogue parliament on Monday for five weeks is helping these efforts.

“Prorogation has created a real visceral anger on the doorsteps,” she said.

As University of Hertfordshire students milled about this medieval abbey town in gowns and mortarboards awaiting graduation ceremonies, members of the public walking by the old clock tower expressed concern at Johnson’s Brexit strategy and what it was doing to the Conservative party.

Ruth Peniket, a long-time Tory supporter and Remain voter from a neighbouring constituency, says the shift from a “moderate Conservative party to an extreme one” would encourage her to come here and campaign for the Lib Dems.

“What has happened in this last week has been horrendous – the expulsion of Conservative MPs, the push for no-deal. It swings me even further behind the Liberal Democrats,” she said.