Jamie Reed, Labour MP for Copeland, announced today that he will be leaving Parliament in January to take up a senior role at Sellafield, the nuclear reprocessing and power plant in his constituency. Mr Reed was one of the most prominent and outspoken critics of Jeremy Corbyn’s leadership of the Labour party.

So you might think that his resignation would be good news for Corbyn. But it isn’t, because Reed’s seat of Copeland is not at all safe for Labour.

Governing parties don’t win by-elections from the official opposition party. Basically ever. It’s practically a rule of politics.

Governing parties can successfully defend by-election seats that they already hold. They can also lose by-election seats; they can lose them to the official Opposition, or smaller opposition parties, or independent local candidates. But they don’t gain new seats at by-elections. It never happens.

Never? Well, rarely. Since the Second World War, the Governing party has won four by-elections from the main Opposition, and I’d argue that only one of them is a ‘normal’ case.

1953 Sunderland South by-election

In 1953, under the second Churchill Government, Conservative Paul Williams won the Sunderland South by-election following the death of the incumbent Labour MP.

The seat was already ultra-marginal. Labour was defending a majority of 306 from the 1951 General Election, when only Labour and the Conservatives stood candidates. In the by-election, the Liberals stood a candidate too, drawing votes away from Labour. Williams won for the Tories, but with a lower vote-share than he got at the General Election. The Liberals’ decision to stand in ’53 seems to have been the critical factor.

1960 Brighouse and Spenborough by-election

Brighouse and Spenborough was a marginal constituency that Labour had managed to just about hold onto throughout five(!) elections and by-elections in the 1950s, though it scraped a win in 1959 by just 47 votes against the Conservative candidate.

In a two-candidate by-election, with the same parties standing as at the general election, Conservative and National Liberal candidate Michael Norman Shaw won with another small majority (666 votes, 1.5%). This happened under the Macmillan government. Shaw went on to lose the seat again at the next General Election.

1961 Bristol South East by-election

Ok, this one really really doesn’t count. In 1961, still under Macmillan, Tony Benn’s father died. Benn automatically became Viscount Stansgate, a hereditary peer and member of the House of Lords. This meant his Bristol East seat in the Commons became automatically vacant.

Benn, though, wanted to be an MP and opposed the institution of the House of Lords. At this time there was no way to renounce a peerage so Benn decided to contest the by-election for his own seat, knowing that he would probably be disqualified afterwards.

Benn won 70% of the vote against his Conservative opponent Malcolm St Clair, but an Election Court later declared that St Clair was the MP.

St Clair resigned voluntarily two years later, after the law changed to allow Benn to renounce his peerage and stand for Parliament. Benn handily won the new by-election in 1963, which was left uncontested by the Tories and the Liberals. The whole affair showed British politics at its honourable best.

Mitcham and Morden by-election, 1982

In the early Thatcher years, Bruce Douglas-Mann was Labour’s MP for Mitcham and Morden. In 1982, he decided to leave Labour and join the Social Democratic Party. However, unlike the other defectors, he also decided he should seek a new mandate and so resigned from the Commons, triggering a by-election which he contested.

Of course, the Labour party selected a new candidate to stand against him.

The seat was already highly marginal. Douglas-Mann had a majority of just 618 over the Conservatives in the 1979 election. He must have known that he had very little chance of winning with his vote split between himself and the Labour candidate, even with the SDP-Liberal alliance meaning the Liberals pulled their candidate.

The Tories won the seat with a vote-share almost unchanged since the 1979 election as the Left vote split between Labour and the SDP.

Four cases in the last 90+ years. One of them (Benn) clearly doesn’t count because Benn won the vote. Mitcham involved a defection and a split vote between a new party. Sunderland South had the Liberals entering and splitting the vote. Only Brighouse and Spenborough is a straightforward case of a by-election gain by a Government against the Opposition (though I’ll give you Sunderland if you think I’m being unfair).

Also note that Sunderland, Brighouse and Mitcham were all highly marginal seats, with none of them having a majority over 700 to defend.

Incidentally, this is why political types find it so sad and funny when Corbyn supporters boast that Labour has ‘won’ by-elections under Corbyn.

Labour has held five seats in by-elections with Corbyn as leader (Oldham West, Sheffield Brightside, Ogmore, Tooting and Bately & Spen). Four of these are rock-solid Labour safe seats, while Bately & Spen is both moderately safe and was not contested by the other major parties following the murder of Jo Cox.

But even if they were marginal seats, oppositions don’t lose by-elections. They just don’t. It’s like boasting that Labour has a website or that it registered its candidates correctly. Holding seats at by-elections is the bare minimum required to be considered a functioning opposition.

But…

Copeland is a marginal seat, with Jamie Reed winning 42.3% of the vote vs Conservative Stephen Haraldsen’s 35.8% in 2015, coming out to a 6.5% Labour lead. It also had UKIP on 15.5% and the LibDems on a poor 3.5%.

And that was in May 2015, when polls showed the Conservatives to be a point or two ahead of Labour. Today’s UKPollingReport Average has a national Conservative lead of 13 points.

Add to this that the UKIP vote seems to have taken a small hit since the election to the benefit of the Conservatives, while the Liberal Democrats are on the rise to the detriment of Labour. And there are other causes of concern:

Labour will also be losing Reed himself a popular local incumbent with some personal vote.

Labour will have to select a new candidate, and all Labour selections nowadays end up as internecine fights between Momentum and the moderate wings of the party.

Copeland borders Westmorland and Lonsdale, the constituency of Liberal Democrat leader Tim Farron. This gives the LibDems a base to organise from and a stock of local activists. And LibDems are good at by-election.

This is not to say that Labour will lose. Oppositions don’t lose by-elections to Governments easily. But it does mean that the party will really have to fight for the seat to be sure.

Labour also has some advantages. The incumbent party gets to move the Writ of Election for a vacant seat, effectively choosing when the by-election will be. Some commentators are already speculating that they might choose to wait a few months in the hope that Theresa May’s polling honeymoon will end.

All the laws of political physics say Labour should hold Copeland. That is the absolute bare minimum anyone should expect.* The fact that we’re even considering the possibility of the Government winning it is, frankly, crazy.

If Labour holds Copeland, that is not a cause for any Corbynite rejoicing or boasting (unless Labour wins it by a stonking impressive majority).

If Labour loses Copeland, or even if it comes close, then that’s a sign that the party is in the deepest crisis that an Opposition party has faced since the Second World War.

*David Herdson helpfully pointed out that Labour could, of course, lose to another Opposition party. That happens much more often than losing to the party of Government. But in some ways, that might actually be worse for Labour. It would set up that other party (say, the LibDems) as the home for discontent voters. It would threaten Labour’s status as the main Opposition party in the political dynamic. I don’t see it happening here, though. The LibDems and UKIP are both too far behind to win.