Jeremy Corbyn cannot have expected a particularly lavish Christmas present from Jamie Reed, the departing Labour MP for Copeland, but he must have hoped for something a bit better than a byelection in one of the worst possible places for Labour to defend in early 2017.

Not only will it offer clues as to how the party is faring in the country, but it also raises the question of how many Labour MPs might be prepared to resign before the next election if a decent job offer comes their way. Reed was particularly outspoken and disaffected, and his constituency is also going to be massively altered in the next set of boundary changes, but others are not much keener on the prospect of serving a few more miserable years in parliament before possible defeat or deselection.

Reed had represented Copeland since 2005, taking over from Jack Cunningham who had held the seat for 35 years from 1970. As a constituency that has been continuously Labour-held since 1935, this coastal slice of west Cumbria may look at first glance like a safe seat, but in the current circumstances nothing could be further from the truth.

Copeland’s long history of Labour allegiance owes a lot to the miners’ vote in the now-closed west Cumbria coalfield, and since the 1950s its position as the centre of Britain’s nuclear industry. Its towns have more than their share of deprivation, aggravated by poor transport links. But it has only rarely produced huge Labour majorities, because there is a substantial and loyal Conservative vote along the coast from St Bees down to Millom, and in the Lake District hinterland. Electoral swings tend to be low in Copeland, with Reed’s majority having gone down only gently since 2005, from 5,157 on new boundaries then to 3,833 in 2010 and 2,564 in 2015, a swing of a little over 3%, compared to the national average of 5% between 2005 and 2015.

Cunningham survived as Copeland’s MP during Labour’s anti-nuclear years in the 1980s, but only with a narrow majority and – like Reed – by identifying himself strongly with the constituency’s nuclear industry. The centre-right within Copeland Labour party has been underpinned by the strength of traditional trade unionism among nuclear workers, who tend to be represented by the GMB union. Both Reed and Cunningham had solid GMB support, an invaluable help in getting selected and retaining the nuclear workers’ loyalty.

Copeland could have been designed to test Labour’s strength among traditional voters whose loyalty to the party appears to have weakened in the last couple of years. It lacks all the demographic groups in which Labour support has been stable or increasing. Hardly anywhere in the country has a lower proportion of students than Copeland, and the constituency is also among the least ethnically diverse in England, with white people comprising 98% of its population. It is a long way from any large city – Manchester is a three- to four-hour train journey from Whitehaven. Well over half the electorate is over 45. Copeland’s leave vote is 10 points above the national average in the Copeland council area that covers over four-fifths of the constituency.

Three parties will be fighting a serious campaign in the Copeland byelection: Labour, the Conservatives and Ukip. It is not normal for the main opposition party to be anxious about holding a byelection seat they have represented for 81 years, but these are not normal times.

Given Labour’s reliance in Copeland on a traditional white working-class vote, and west Cumbria’s perennial sense of being neglected and isolated, it is a dream target for Ukip under its new leader Paul Nuttall, who has prioritised going after the Labour vote in the north. There is room for improvement from his party’s showing in 2015 when they polled 15.5% and finished third. Winning the seat would require a big bandwagon effect to develop during the campaign, but the swing required is about what the Lib Dems achieved in the high-income remain-voting constituency of Witney, and well short of the turnover of votes in Richmond Park.

The net effect of Labour losing a chunk of its vote to Ukip would probably be to enable the Conservatives to come through the middle and win this seat where they have often come fairly close – as long as their vote is loyal. If the Conservatives win, it would be a rare example of a government party gaining a seat in a byelection. The last time this happened was in Mitcham and Morden in 1982, and before that the only two previous postwar cases were in 1960 and 1954. Having lost Richmond Park, Theresa May could do with an unexpected addition to the Commons majority.

A Labour hold would require a good candidate and an enthusiastic campaign, and that is certainly possible, as has been shown in some rather different constituencies such as Sheffield Brightside and Hillsborough and Oldham West, where Ukip has flopped. But it would provide only temporary relief to the party. Copeland’s geography of deep water and high mountains gives us some rich metaphorical terrain for writing about this bare-knuckle fight of a byelection.