A Labour MP has quit the Commons, triggering a byelection that will severely test the party’s performance in its traditional “red wall” heartlands. The Guardian understands that Mike Hill resigned as MP for Hartlepool while a parliamentary investigation continues into his behaviour, and that party leader Keir Starmer told him he needed to quit some time ago. Northern Tories are confident they could clinch the seat and add it to their list of constituencies that have for the first time elected a Conservative MP. One Labour source said Hill “has done the right thing and resigned” and that “it is right above all else that any investigation is carried out fully and that justice is done”. The date floated by Labour figures as the most likely for the byelection to go ahead is 6 May, in line with other local elections. Hill, 57, has been the MP for Hartlepool since the 2017 snap election. He held on to the seat in 2019 but saw his majority halved – and was likely saved from the Tories taking it by the Brexit party standing and splitting the vote in an area that voted 69.6% to leave the European Union in the 2016 referendum. One Labour HQ worker laid bare the challenge facing the party to hold on to the constituency, admitting: “It’ll be really tough. Maybe if we can convince Jeff Stelling to stand, we’d have a shot.” Another party insider said: “You don’t have to be a maths genius to work out it’s going to be tough for us.” While byelections are often challenging for an incumbent government, especially given the Tories have been in power for 11 years and considering the criticism ministers have faced for their handling of the coronavirus pandemic, the upending of decades of traditional political dogma in the 2019 election means Conservatives believe Hartlepool is now in their grasp. “We should win it,” one northern Tory MP said, while another rated the party’s chances as “very good”. But they said that uncertainty remains until Labour announce who their candidate to pick up the mantle from Hill is. Laura Pidcock, the former MP for North West Durham who herself fell victim to the Conservatives’ surge in northern England, is one of the names being discussed. Another is Paul Williams, another former Labour MP, wiped out by the Tories’ surge into the red wall in 2019, who is standing to be Cleveland’s police and crime commissioner this spring. A GP, he is thought to be the favoured choice of the local party. One Labour source said: “He’s a frontline doctor in a town that’s had its hospital cut. It’s a pretty obvious fit.” But Conservatives say they have already collected records of his vocal support for another referendum posted on social media in the run-up to the last election. If he is selected, one Tory backbencher said gleefully: “We’ve got a library full of those.” Labour sources refused to be drawn on who will be picked. “There will be a party process,” they said. Hill, who has not yet spoken publicly about his resignation, is at the centre of an ongoing parliamentary investigation over an allegation of sexual harassment, which he has strongly denied. He was suspended from the Labour party during an investigation in 2019 but the whip was reinstated a month later. It is understood that a tribunal hearing over the matter is scheduled for May, the same month as the local elections. In January last year, Hill failed in a bid to stop his name from being revealed in reporting of a forthcoming employment tribunal case taken by a woman who has accused him of sexual assault and harassment. Hartlepool’s Labour party has suffered a damaging split in recent years, partly due to disagreements between the Jeremy Corbyn-supporting left and the moderates. It has lost control of the town council, now run by former Brexit party councillor Shane Moore, and its number of seats has dropped from 18 to only six on the 30-strong council.