Jeremy Corbyn has attempted to play down Labour’s general election strategy shake-up for the last two weeks of the campaign as the party pivots towards trying to save vulnerable heartlands in the north and Midlands.

The Labour party leader insisted he would take the same anti-austerity message to all parts of the country at his “plan for nature” launch in Southampton. Yet, behind the scenes, strategists are instigating a switch from going after Tory marginals to consolidating seats they already hold and trying to win back leave voters.

The party suffered a bruising result in the YouGov MRP poll, which put Labour on just 211 seats, its lowest since 1983.

“Our campaign is in every part of the country. I am travelling all around the country. I say the same thing at every place I go. I don’t have one message for one group,” Corbyn said, addressing questions that his campaign is about to change course.

“I say the same message everywhere. Vote Labour in order to get a government that will deal with the inequalities and poverty and injustice austerity has heaped on this country.”

But the Guardian understands that after beginning the campaign with a resolutely offensive strategy, the party recognises that it now has much more to do to hold constituencies in Brexit-voting areas.

It plans to hammer home the benefits of a pared-down set of its manifesto commitments that will put money directly in the pockets of voters in these constituencies.

Labour strategists also say they will focus relentlessly on the risk of returning Boris Johnson to No 10 for five years, and seek to make the election a binary choice between Labour and the Tories in the hope of squeezing the Liberal Democrat vote further.

Q&A What is ‘tactical voting’? Show Under the first-past-the-post voting system, tactical voting is when you vote for a party that you would not normally support in order to stop another party from winning. For example, in a constituency where the result is usually tight between a party you dislike and a party you somewhat dislike, and the party you support usually comes a distant third and has no chance of winning, you might choose to lend your vote to the party you somewhat dislike. This avoids ‘“wasting” your vote on a party that cannot win the seat, and boosting the chances that the party you dislike most will lose.

Even before Wednesday evening’s YouGov projection, which gave Boris Johnson a 68-seat majority and predicted Labour would lose 44 seats, party strategists were concerned about the imperative of appealing to leave supporters.

They point out that even in the 2017 general election, the six seats Labour lost were all in leave areas; and, even in those they held, the increase in Labour’s vote share was significantly lower than elsewhere.

The YouGov projection, which showed the Lib Dems failing to make inroads in many of the seats Labour is targeting, has strengthened the hand of those who have been making that argument internally. These include the “four Ms” – Seumas Milne, Andrew Murray, Len McCluskey and Karie Murphy, Labour sources suggested.

It was McCluskey, the general secretary of Unite, who warned against expanding freedom of movement, as some Labour activists had hoped to.

In traditional Labour heartlands the reaction among candidates to a new focus on their constituencies was mixed.

Great Grimsby candidate Melanie Onn said she had not had a single visit arranged centrally to help her defend her 2,565-majority in her heavily leave-voting seat, so was surprised to hear about a shift in focus to the north of England and Midlands.

“I’ll just have to wait and see what this refocus is, I’m not really sure what it means,” she said.

The shadow health minister, Jon Ashworth, appears to be acting as a bridge between Corbyn, his shadow cabinet and those candidates in northern and Midland constituencies with a leave majority.

As well as helping Onn launch her campaign he has also visited Stoke, which voted 72% leave, for a joint event in support of Labour candidates there.

Sonya Ward, who is trying to take back the traditional Labour heartland seat of Mansfield from Tory Ben Bradley, said she had been frequently visited by the shadow cabinet – including Corbyn. She said: “I’ve been pretty well supported to be honest.”

One Labour source in a heartland seat said: “I think they have seen the polls, after ignoring the polls for a long time. Six months ago I started saying we were losing our vote. To be honest I think they’ve realised a little bit too late that they’ve been more interested in trying to save Emily Thornberry’s seat in Islington than Labour heartlands.”

Labour’s Barry Sheerman, defending Huddersfield, which voted 51% leave in the referendum, said he would welcome more support for his colleagues in Yorkshire.

While he has a healthy 12,000-majority, some female politicians elsewhere in the region risk losing their seats. The Tories are predicted to pick up three seats in South Yorkshire, including Don Valley, Penistone and Stocksbridge and Rother Valley, and four in West Yorkshire including Dewsbury, Keighley and Wakefield.

He said: “I think anything that targets any vulnerable seats where we have got absolutely first-class MPs – some of whom are the leading female MPs in parliament – is important, so any help that can be brought to them I would really welcome.”