Bad weather in Copeland and a resurgent Conservative campaign in Stoke will make it a challenge for Labour to win both of Thursday’s byelections, local campaign leaders for the party have said.

In an attempt to dampen expectations of a clean sweep, the Stoke campaign chief, Jack Dromey, said the race there was a “three-way marginal” where the Labour candidate, Gareth Snell, was under threat from the Conservatives’ Jack Brereton as well as Ukip’s Paul Nuttall. “The Labour campaign has been hard-fought and there is significant progress,” said Dromey, a Birmingham MP. “But it is a tough, tough, tough marginal.”

Dromey said the result would be a “defining moment” in the British political landscape, though turnout in the seat has been historically one of the lowest in the country. “It is a defining moment, it is Ukip breaking through in Labour’s heartland or it is the turning of the tide on Ukip,” he said.

“The backdrop is a raft of discontent felt by the people of Stoke. There is an attempt by the grotesque populism of Ukip to exploit that discontent, which they have no answer for.”

Labour has never lost Stoke-on-Trent Central since it was created in 1950. In 2015, Tristram Hunt held the seat with a majority of 5,179, while Ukip beat the Conservatives into second place.

Dromey said that the impression on the ground was that the Ukip campaign was in disarray, but the situation was fluid. Defence, as well as doubts about Labour’s patriotism were key issues, he said, but the party had tried to “confront that head-on” during the campaign, including with a leaflet emblazoned with a St George’s flag.

“We brought George Robertson the former director general of Nato, who came to say a Labour government founded Nato, that Labour is the party of strong defence,” he said. “Many people in Stoke have relations in the army, you meet old soldiers, army cadets. Pride in country is something we should never ever cede to the right in politics.”

In Copeland, Labour’s elections co-ordinator Andrew Gwynne MP said the geography and the weather forecast meant the party could have difficulties on election day. Storm Doris, which will bring heavy rain and gale-force winds, would make getting the vote out tough for activists.

“Labour support is there and our challenge is to get them out,” he said. “And it’s a big challenge, it’s a very isolated constituency. If we can get people to come out, we are in with a shout. That will be the difference.”

Labour won the seat with a majority of 2,564 in 2015. One source who has campaigned in the area said any hope of keeping the seat was down to the threat to local NHS services. Labour’s Gillian Troughton, a former local doctor, has fought a campaign based on the closures of maternity services, against the Tories’ Trudy Harrison. The Conservative campaign has been fixed on the local nuclear industry, which the Labour leader, Jeremy Corbyn, has historically opposed.

“We would be in a different place without the NHS issue in Copeland,” the source said. “We are 18% behind in national polls, and the reason we are even still in the race is because of the NHS.”

Gwynne, who was recently promoted to his elections role, said one thing the party could learn from Copeland was to address issues beyond the EU. “It’s two issues, the NHS and nuclear,” he said.

“And it’s two parties in with a chance. Brexit hasn’t featured at all. We get hooked on certain issues in Westminster, and it’s not to play down some of the issues that led to the leave vote, but there is much more that concerns people.”

On Thursday, Corbyn and the shadow Brexit secretary, Keir Starmer, will host a summit for progressive European parties, including Gianni Pittella, the leader of the S&D group in the European parliament. Corbyn will follow that with a speech on Friday on the party’s vision for a post-Brexit Britain and the protection of jobs and the economy.

One senior Labour source said they believed that win or lose, there would be relatively little soul-searching afterwards. “As far as the leadership is concerned, if they win both, it will be a sign that the national polls are wrong, even if we only scrape a win when we should be increasing our vote share,” the source said. “If they lose, it will be because of the coup. That’s the only thing they will hear because that’s what their supporters respond to.

“There will definitely be relief if we win, but I can’t see any result really prompting a change in direction because that’s not how they operate.”