Keir Starmer has said he will take “full responsibility” for the result of the Hartlepool by-election as an opinion poll showed the Tories opening up a double-digit lead in the constituency.
The Labour leader said the party would be "fighting for every vote" and that he hoped it would not lose another seat in the Red Wall.
But he admitted Labour still had "a mountain to climb" after its devastating defeat under Jeremy Corbyn to be in a position to oust the Tories from power.
Ahead of Thursday’s vote, a Survation poll for ITV’s Good Morning Britain put the Conservatives on 50% – 17 points ahead of Labour in a seat it has held since it was created in 1974.
The scale of the challenge in the seat is not a surprise to many in Labour as the Brexit Party received a chunk of the vote in 2019 and popular local Tory mayor Ben Houchen has brought investment to the region.
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Mr Starmer's bigger test will be how Labour does in council and mayoral elections across England, as well as national polls in Scotland and Wales.
"I hope we won't lose Hartlepool. We are fighting for every vote there. I know that every vote has to be earned," he told BBC Radio 4's Today programme.
“I said on the day that I was elected (Labour leader) that it was a mountain to climb. It is, we are climbing it and I’ve got a burning desire to build a better future for our country.
"I don’t think anybody realistically thought that it was possible to turn the Labour Party round from the worst general election result since 1935 to a position to win the next general election within a period of one year.
“It was always going to take longer than that.”
He added: "I take full responsibility for the results, just as I take full responsibility for everything that happens in the Labour Party under my leadership."
Some on the Labour left have criticised Mr Starmer for failing to set out an alternative vision for the UK that would challenge that of the Tories.
While the Government has been mired in sleaze scandals and rows over Covid contracts, it has also attempted to move its tanks onto Labour’s lawn.
The Tories have appointed a “levelling up” tsar to help deliver on their 2019 manifesto pledge, following concerns that people struggle to understand what the phrase means.
Tory MP Neil O’Brien has been tasked with overseeing the programme that many 2019-intake Tories from former ‘Red Wall’ seats believe is critical to their re-election.
No 10 announced on Tuesday that it would publish a ‘levelling-up’ white paper to set out more policy detail later this year.
Ministers have been accused of channeling funds to Conservative seats rather than into areas which are in most need of economic and infrastructure support.
On a campaign visit to Wales, Mr Starmer insisted that Tory sleaze allegations should matter to voters - and showed there was one rule for Boris Johnson, and one for everybody else.
“I don’t actually go along with this argument that says, ‘well it’s priced-in, it may be that the Prime Minister is not being straight but it doesn’t really matter’,” he said.
“Being the Prime Minister of the United Kingdom is an honour. It’s a privilege and we should not ever accept that it’s priced in.”
