MP Jamie Reed has been a constant social media thorn in the side of the Labour leader.
He resigned as shadow health minister via Twitter even before Mr Corbyn had finished his leadership acceptance speech last September.
He called on him to step down in a long letter just after the EU referendum that included the suggestion that Mr Corbyn and his supporters had injected an “unprecedented poison” into the party.
The big fact, though, is that the constituency is being abolished under the terms of the boundary review.
This might have an impact on the calibre of candidate – who will be willing to fight a seat that will only exist for a maximum of three years?
That Mr Reed thinks he can serve his constituents better as an executive at Sellafield, rather than as an MP, is a remarkable statement.
But his antipathy to Mr Corbyn comes as no surprise. His leaving present, though, is a scientific testing ground for the theory that he can turn a mass membership party into actual votes, and seats where they matter.
Losing a seat that has been held constantly (including in its previous incarnation as Whitehaven) should be totally unthinkable. But with the Conservatives just 2,564 votes behind at last year’s General Election and currently 8-10% ahead in national polls, Copeland is definitely “in play”.
The seat voted 62% to leave the EU in the referendum. But this is not as simple as it seems. UKIP did well to get 6,148 votes last May, but it is not perfect UKIP territory given the strength of the Conservatives here.
UKIP and the Conservatives risk splitting the “Brexit means Brexit” vote, too. But if the Conservatives run a hard UKIP-tinged campaign this could turn off a minority of its own pro-Remain supporters.
Labour’s challenge is acute, though: maintain its base of support, win back some Labour supporters who voted Leave, but also provide a home for 14,000 Remain voters in Copeland.
Above all, the presence of Sellafield puts a special dynamic on this election. Labour will surely have to get a pro-nuclear local union official as its candidate, which is contrary to the instincts of the leadership.
Local health issues might also end up as important as Brexit as there have been performance issues in local hospitals.
That said, the by-election campaign will be against a backdrop of Article 50 debates in Parliament and the Supreme Court decision. It will settle the issue of how defined UK voters are by Brexit.
It is all to play for in Cumbria in early 2017.
POLITICAL EDITOR,SKY NEWS