Laura Reineke has been a mermaid for seven years now. Or more precisely, she’s a member of Henley Mermaids, the name she and a few friends from her open-water swimming club gave to the WhatsApp group they created for arranging river dips. Seven years of navigating various unmentionable waterborne substances later, the mermaids aren’t just swimmers now but fully fledged clean-water activists, campaigning to highlight pollution in Oxfordshire’s waterways alongside the likes of TV presenter Steve Backshall (who lives nearby with his Olympic rower wife Helen Glover) and lobbying local politicians. Reineke, who works for the conservation charity Wild Fish, still swims daily with the help of an app tracking Thames Water’s regular discharges into the river: but lately, she says even the supposedly clean stretches seem murkier. “You can’t see the bottom any more, the plant life is covered in sewage – it’s grotty. It’s really, really sad.” Though as she points out, it’s much worse for the fish. When the Liberal Democrat leader, Ed Davey, launched his campaign by falling off a paddleboard into Windermere, he was trying to dramatise an unexpected wedge issue at this election: clean water. His party is targeting a ribbon of ostensibly safe Tory seats stretching west along the Thames, from Maidenhead (where Theresa May is retiring) to Boris Johnson’s old seat of Henley and Thame (where Rishi Sunak visited a rowing club this week, only to be photobombed by a boatload of Lib Dem activists launching their national campaign on sewage dumping) via commuter belt Didcot and Wantage to David Cameron’s old Witney seat, now redrawn on new boundaries. If a political earthquake is really coming in July, the tremors should be detectable here. Water pollution might seem a niche issue to some. But in the still largely prosperous rural Tory heartlands, filthy rivers have become a surprisingly powerful symbol of national decline; a way for the Lib Dems at least to open conversations in the kind of “blue wall” seats that Labour will never win but which stand between Keir Starmer and a landslide. For the 15 years I’ve lived in south Oxfordshire, first in Witney and then in Didcot and Wantage, the electoral war was invariably happening elsewhere. Parties fight over marginal seats, not safe ones where nothing ever changes. But this time is different. Boundary changes and new estates springing up catering to younger families moving out of London have made things less predictable, as has the undercurrent of utter exasperation in everyday conversations. ‘Waitrose woman’ – the stereotypical affluent voter the Lib Dems are targeting across the home counties – has been profoundly unimpressed by shabby conduct during Partygate, by incompetence and infighting, by rising taxes and bills with precious little to show in return (not least from Thames Water, now begging for a government bailout). The Tories lost control of their last remaining Oxfordshire district council last year, and this week’s shock Survation MRP poll forecast all four of these riverside seats turning a scarcely conceivable yellow. Nigel Farage’s return to the national stage, meanwhile, leaves Rishi Sunak with an impossible choice: if he responds by lurching rightwards, that would be a turnoff in places like this whose conservatism is of a gentler, remain-voting kind. Charlie Maynard is standing in an industrial estate on the banks of the River Windrush, a Thames tributary that winds behind the honey-coloured stone buildings of Witney’s high street, when I track him down by phone. He’s the kind of candidate – background in finance, seat on the local council, pictures on his website of the family labrador – that the Lib Dems invariably select for seats like this one, which has been Conservative for more than a century. “There are so many Tories here who say, ‘I’ve voted Tory for ever and I’m not doing it this time,’” says Maynard, who lists what he hears on the doorstep: worries about the cost of living, inability to get a dentist, schools and filthy rivers. “Sewage is the big one. People are just like, ‘How can this be happening?’ I think it does speak to appalling government management and regulation in the longer term.” He’s also lost no time pointing out on doorsteps that the 22-year-old Labour candidate resigned a few days into the campaign. Given the Conservatives’ Robert Courts had a solid 15,177 majority in 2019, winning here would still be a huge stretch for the Lib Dems, even with new boundaries. Across Oxfordshire, they need a meltdown in the Conservative vote plus highly efficient tactical voting in seats with no established tradition of it. Blue wall Tories’ best path to victory may well be straight through the middle of an angry but confused electorate that is unsure exactly how to get them out. The progressive thinktank Compass is running a Win As One campaign nationally to encourage tactical voting by Lib Dem, Green and Labour activists, with local grassroots groups working to spread the message in potentially winnable seats from Kent to Cumbria, Surrey and Sussex. (Compass Oxfordshire is running street stalls locally identifying the Lib Dems as the best hope in Witney, Didcot and Wantage, Henley and Thame, in Banbury, in north Oxfordshire, they’re backing Labour.) Yet this support comes with strings attached. Compass only backs candidates who support proportional representation, helping small progressive parties to break through. (PR would arguably be a godsend for Reform too.) “Every five years we end up in this situation where parties say, ‘Lend us your support’, but there’s no return for the voters,” says Luke Hurst of Compass, who argues that tactical voting tackles the symptom but not the underlying democratic problem. Can a progressive alliance really help pull off something extraordinary in 2024, a revolt wiping the Tories out in what used to be their home turf? The risk is always that what looks mathematically possible on a pollster’s spreadsheet doesn’t account for human nature, with all its tribal instincts and unexpected allegiances. In Henley, Laura Reineke knows the Lib Dems are the tactical choice, but she can’t bring herself to do it: instead she’s backing her fellow mermaid, Jo Robb, standing for the Greens. The lingering doubts many still harbour about both Sunak and Starmer, meanwhile, mean that with four weeks of the campaign still to go those confident polling forecasts of a landslide feel decidedly premature. But for the Tories, even to be having to spend resources defending seats like this is unusual. Whatever the results in Oxfordshire, what’s happening now feels like the last act of the Brexit drama; the curtain falling on a party torn apart by David Cameron’s disastrous referendum, then temporarily patched together by Boris Johnson’s unstable victory, now finally unravelling. Even, perhaps, in the places where both careers began. Gaby Hinsliff is a Guardian columnist