By explicitly calling for Britain to rejoin the EU yesterday, Wes Streeting was making life as hard as possible for Andy Burnham.
In Makerfield, where the Manchester mayor hopes to fend off Nigel Farage's Reform UK on June 18, 66 per cent of local voters backed Brexit in the 2016 referendum.
With outgoing Labour MP Josh Simons passing on a majority of just 5,300, the mayor will not welcome a spotlight being shone on his own anti-Brexit views.
If Mr Streeting is to have any hope of winning the leadership, he will need to target southern, Remain-supporting voters who are increasingly migrating to the Green Party and the Liberal Democrats.
He regards the pro-Brexit, Red Wall seats in the North and Midlands as largely a lost battle. Mr Burnham does not have that luxury if he is to win Makerfield, re-enter the Commons and claim No 10.
Last night, government sources – not affiliated to any leadership camp – expressed their fury at Mr Streeting's intervention, with one saying: 'Wes claims to be a unifying force, but he has done this for his pure, selfish ambition.'
As Mr Streeting confirmed he will stand in any contest to replace Sir Keir Starmer as Labour leader, the former Health Secretary said: 'Leaving the European Union was a catastrophic mistake. The biggest economic opportunity we have is on our doorstep. We need a new special relationship with the EU, because Britain's future lies with Europe – and one day, one day back in the European Union.'
After Mr Streeting's announcement, Mr Farage said: 'The North will be lost to Labour for ever.'
Nigel Farage said the North would be lost to Labour forever after Wes Streeting said Brexit was a 'catastrophic mistake'
One Whitehall source said: 'In the local election, voters sent a clear message: they are frustrated with a political system they no longer feel serves them.
'Wes's response? To attempt to reopen the most divisive debate in modern British politics and say that he would turn the next general election into a de facto second referendum.
'That is not leadership, and it isn't uniting our country. It is desperate self-interest – and a total betrayal of Labour values.'
Another source said: 'This is the one policy that uniquely serves to divide Britain and Wes has reached for it straight away.
'It looks like he seeks to impose five years of misery on the country. He claims to be a uniting leader but his first impactful thing has been to choose something to divide the country.'
The source added: 'This is a desperate attempt to shore up support based on identity politics and a culture war which frankly most people gave up on many years ago. Student politics.'
Mr Streeting told supporters at a fringe Labour event yesterday that any move towards rejoining the EU would require a fresh mandate from the public. He said: 'We do have to respect the fact that we left the European Union through a democratic choice, so we will need to, in future manifestos, seek a new mandate.
Mr Streeting said on Saturday he would be a candidate if a Labour leadership race were to be triggered
'I'm not proposing that we break a manifesto that we've stood on at the last general election, because frankly, trust in politics and politicians is in even shorter supply than money in our country.'
He added he would be 'proud to campaign for Andy Burnham' in the by-election, saying: 'We've got to make sure we deliver the change people wanted to see, that we go into the next election with an equally bold and ambitious plan.'
He also attempted to frame his campaign as a contest of ideas rather than personalities, stating: 'We need a proper contest where all candidates can put their best foot forward.
'It's got to be a battle of ideas, so that whoever wins comes out strong before it.' Acknowledging the threat posed by Mr Farage, he said: 'For the first time in Labour's history, our biggest opponents aren't the Conservatives, but Nigel Farage and Reform.
'Scottish and Welsh nationalism represents an existential threat to the future integrity of our entire country, but the English nationalism represented by Farage and Reform is a threat to the values and ideals that have made this country great.
'Progressives understand this threat, and they want us to confront it, but they are increasingly losing faith the Labour Party is capable of rising to the challenge.'