St Albans was in celebratory mood last week, its streets thronged with young graduates in gowns and mortarboards for daily graduation ceremonies.

At the medieval abbey in the heart of the tiny city, erstwhile University of Hertfordshire students – most looking faintly uncomfortable in formal dresses and suits beneath their rented outfits – queued with proud families around its stone walls for good seats beneath its magnificent arches. A few yards away, a bar advertised “free prosecco for gowns”.

But – as elsewhere in this fractured country – below the surface there was incomprehension, incredulity and simmering anger.

“I voted Leave in 2016, but the way Boris Johnson is going about things makes me wish I’d voted Remain. He’s a bulldozer,” said Clive de Carle, 56. “If we have an election, I’ll vote Lib Dem. Hopefully, that will mean one less Conservative MP, and they might get the message.”

Views such as De Carle’s could deliver a dramatic outcome to a general election whenever it might take place in the coming weeks or months. Brexit is the issue that will determine how millions of people cast their votes – and, as data unveiled by the People’s Vote campaign on Sunday shows, tactical voting could have a profound impact on the result.

The People’s Vote has identified scores of constituencies around the country in which Remain supporters could set their usual party political preference to one side in favour of the pro-Remain candidate best placed to win the seat, with the aim of fortifying the pro-Remain cohort in parliament.

St Albans is a case in point. The constituency has usually been held by the Conservatives, although an earlier instance of tactical voting produced a win for New Labour in the 1997 landslide that swept Tony Blair into No 10.

Since 2005 the seat has been held by Anne Main, a pro-Brexit Tory and supporter of the hardline European Research Group. Even though the area voted Remain by 63% to 37% in the 2016 referendum, Main was re-elected the following year, albeit with a majority cut in half to just over 6,000.

Daisy Cooper, Liberal Democrat candidate for St Albans, pictured in 2017 with then party leader Tim Farron. That year, Lib Dems cut the Tory majority in half. Photograph: Chris J Ratcliffe/Getty

But, crucially, Main’s 43% share of the vote was easily outweighed by the combined 57% share of the Lib Dems, Labour and Green parties. This time round, could those voters unite behind one candidate?

Daisy Cooper, the Lib Dem contender, is hopeful. “There is now a visceral sense of anger among Remain supporters,” she said over ginger tea at the St Albans museum cafe. “For a long time, people have been frustrated and disappointed, but the last two weeks have made people incredibly angry.

“The prorogation of parliament, the expulsion of long-serving MPs – we’re seeing more and more traditional Tory supporters coming over to us. They feel they can’t trust Boris Johnson, he’s a liar; people like them have been purged, he’s hijacked democracy, and the Conservative party has now become the Vote Leave party.”

But to have a chance of winning, Cooper needs to win the support of local Labour and Green supporters. All parties are wary of coming out in support of tactical alliances, or non-aggression pacts, ahead of any agreement, fearing that it would weaken their bargaining position. But the Lib Dem victory in the recent Brecon and Radnorshire byelection, after the Greens and Plaid Cymru stood down their candidates, has bolstered the case.

Last month the Labour MP Clive Lewis backed a pro-Remain alliance. “Because the Lib Dems are predominantly targeting Tory seats, it becomes quite simple for the Labour party. These are seats the Labour party has no chance of winning … so what we should do is to stand a paper candidate in those seats, which means you put up a candidate by name only and don’t do any actual campaigning,” he said.

I'll probably vote Lib Dem in the next election. Any kind of Brexit will be disastrous, and we've got to do what we can to stop it

St Albans Labour party could not be contacted for comment, but Simon Grover of the Green party said that he would welcome a “genuine conversation” among pro-Remain parties. “I don’t think we’d simply stand aside without a reciprocal arrangement nationally or regionally, but we’re open to talking about it,” he said. The local party is due to discuss the issue of a tactical alliance at a meeting this week.

Cooper said she would also welcome a conversation with the Greens, but an alliance with Labour was “very tricky”. “The biggest hurdle is that they’re not a Remain party. But there is a distinction between the party and Labour voters. It may not be possible or desirable to do a deal with the party, but we are heavily targeting their pro-Remain supporters.”

On St Peters Street, the city’s main drag, Fiona Smith, 62, a former teacher who has lived in the area for 30 years, said she had hovered between the Lib Dems and Labour in previous elections.

“I’ll probably vote Lib Dem in the next election. I think there’s increasing support round here for the Lib Dem candidate. Any kind of Brexit will be disastrous, and we’ve got to do what we can to stop it.”

The biggest battle may be to get people to vote at all. Rob Stone, 32, a barber, was in favour of Remain, but didn’t vote in the referendum because he didn’t think it would be so close.