But what if Corbyn isn’t to blame? You can put it down to other factors, right?
So here are all the other things that people are blaming Labour’s defeat on:
Tony Blair and Peter Mandelson
Shadow chancellor and Corbyn ally John McDonnell told ITV:
What’s interesting is that the overwhelming number of members of the party, the majority of the party, are saying 'unite'.
And we can’t have a situation like we did last week when Tony Blair comes out and attacks his own party, Peter Mandelson as well.
So we are saying to those people 'unite' because people will then see the real Labour party campaigning. And we will win back places like Copeland.
The nuclear industry
McDonnell also said that voters:
...weren’t convinced that the party supported the nuclear industry.
The Sellafield nuclear plant is of course in the Copeland constituency, making it a big issue in the by-election.
Obviously, this is a big one. BBC elections expert Professor John Curtice has written in the Guardiansaying Labour is not paying enough attention to its remain-voting key support.
Labour seems to have decided in recent weeks that its first priority is to stave off the threat from Ukip to its traditional working-class vote, much of which supposedly voted to leave in the EU referendum.
But in so doing it seems to have forgotten (or not realised) that most of those who voted Labour in 2015 – including those living in Labour seats in the North and the Midlands – backed remain.
The party is thus at greater risk of losing votes to the pro-remain Liberal Democrats than to pro-Brexit Ukip.
Not focusing on local NHS issues
John Woodcock, Labour MP for neighbouring Barrow and Furness, told the Today programme:
This was a campaign where we had an absolutely solid NHS issue that really cut through on the doorstep, the future of the local maternity unit and A&E was very much on people’s minds.
It was in many ways a classic byelection issue which could unite the community.
But we failed to do so.
Tories want to move our hospital services over an hour away to Carlisle. Only a vote for Labour TODAY can save our… https://t.co/bN7qrlSari
— Gillian wear a mask & protect others (@Gillian wear a mask & protect others)
In case you hadn’t noticed, Labour has had a rough year or so. And John McDonnell told ITV;
You learn lessons from these things. And one of the lessons you learn is people will not vote for a divided party.
For the last 18 months, 20 months, we’ve been involved in two leadership elections.
So, understandably, in the leadership election, those divisions will come out.
The political establishment and the failed political consensus
Corbyn himself took to Twitter following the Copeland result, saying:
Or... how about blaming no-one, and calling it a great achievement?
Labour MP Cat Smith told ITV:
To be 15 to 18 points behind in the polls and to push the Tories to within 2,000 votes is an incredible achievement.