Josh Simons, the former Makerfield MP, who stood aside for Andy Burnham, has warned that Sir Keir Starmer can expect a "rough time" on the campaign trail.
The Manchester Mayor confirmed he will be running for a Labour leadership bid to "save the party" if he is catapulted back into the heart of Westminster.
Mr Simons announced he would step down to pave the way for Mr Burnham to return to Parliament.
The Prime Minister has publicly thrown "100 per cent" of his support for the Westminster wannabe in the upcoming Makerfield by-election - despite the threat to his future leadership.
Reacting to the pledge, Mr Simon told the Times: “He’ll have some rough doorsteps. Wiganers don’t hold back on what they think.
"But you know, it’s never a bad thing to hear straight from voters exactly what they think about the job you’re doing. So, from that perspective, good luck."
He admitted to having been “frustrated” with Sir Keir's premiership for some time.
When Mr Simons announced that he would resign and trigger a by-election, the outgoing MP hit out at Westminster's watch over the "managed decline of towns" like those in Makerfield.
The Makerfield by-election will take place on June 18
|GETTY
"We have talked big, then acted small, stuck in a politics of incrementalism that cannot meet the moment," he added.
"We have lost the trust of those our party was built to serve. It is my unwavering belief that nothing short of urgent, radical, courageous reform will make a difference.
"That must start with a change in leadership."
The MP's bold statement followed more than 100 MPs signing a statement declaring that it was "no time for a leadership contest".
POLITICS LATEST:
Mr Streeting resigned over a fortnight ago - but did not trigger a leadership contest
|GETTY
Sir Keir was handed the hefty task of calming his mutinous MPs after more than 80 lawmakers urged Sir Keir to step down or set out a timetable for his departure.
Several parliamentary private secretaries, including a number of Wes Streeting allies, resigned from their roles.
Former Safeguarding Minister Jess Phillips was among the most influential resignations - until leadership rival Mr Streeting quit as Health Secretary.
In a damning resignation letter, the Ilford North MP eyeing up the top job accused Sir Keir of a "vacuum" in vision and allowing the Government to "drift".
But he stopped short of announcing he would challenge the former human rights lawyer just yet, insisting that he would wait for Mr Burnham to hightail it to Westminster.
Only then, he claimed, when the "best players are on the pitch", should a leadership challenge be formally triggered.
Meanwhile, the Mayor of Greater Manchester has insisted that Makerfield was not a "stepping stone" in a bid to oust Sir Keir from Downing Street.
At his campaign launch last week, Mr Burnham said: "Somebody has said in this campaign that this is a stepping stone for me. How can it be a stepping stone if it takes you back to where it all began?"
"The reason it comes back is because I’ve never stopped what I started 25 years ago. I’ve never stopped that. I fought for these people in these places as a Member of Parliament. I fought for them as a minister."