With tomorrow’s byelections, Bill Shorten faces what journalists intoxicated by the blood sport of leadership speculation like to call a “crucial test”.

Should he fail, he will carry the millstone of being “the first Opposition Leader to lose a seat at a byelection since 1920”.

It is indeed the case that the Kalgoorlie byelection of that year was unique among the 152 byelections held since federation in being won by the government party at the expense of the opposition.

Nor can it be denied that it will say nothing good about Shorten’s electoral appeal if either Longman, in Brisbane’s outer north, or Braddon, covering north-western Tasmania, falls to the conservatives tomorrow.

However, a bit of perspective never goes astray, and may well be lacking from accounts of the unprecedented disaster Shorten will have led Labor to if things don’t go his way.

Short of holding him personally responsible for Labor’s section 44 failures, it can only be counted as bad luck for Shorten that he faces two byelections in fragile marginal seats.

These days especially, most byelections are non-events, being held in safe seats after senior figures bail out in response to reduced circumstances for their political careers.

The last time a federal opposition had to defend a seat as dicey as Braddon or Longman was in November 1983, when a Liberal Party under the leadership of Andrew Peacock defied Bob Hawke’s stratospheric popularity to retain the Brisbane seat of Moreton with a slight positive swing.

A dozen politicians have gone through the wringer of the opposition leadership since then, in most cases unhappily.

One was Simon Crean, who never got to fight a federal election, and oversaw a defeat in the normally safe Labor seat of Cunningham in October 2002 – the one electoral test he faced in almost two years as leader.

However, this is deemed not to count for Bill Shorten immolation purposes, as it came at the hands of the Greens rather than the Coalition.

The fairest way of putting tomorrow’s results into context will be to compare the swings with the historic norm.

Sixty-seven byelections held since 1949 have produced clear two-party preferred results, with the average result being a swing of around 4% to the opposition.

However, the spread of these results has been very wide, and swings to the government have not been entirely unusual – to the extent that around one in five byelections appears to swing with enough force to knock over Labor’s 0.8% margin in Longman.

The diversity of results illustrates the point that no two byelections are alike, and that local peculiarities can have at least as much influence as the standing of the national leaders.

Longman is a case in point, thanks to the locally potent One Nation factor.

Labor’s win in 2016 was only its second since the seat’s creation in 1996, and was partly owed to the fact that One Nation came gunning for Liberal National Party member Wyatt Roy.

The situation this time is reversed with interest: One Nation is focusing its attacks squarely on Bill Shorten, directing preferences to the LNP rather than Labor, and polling strongly on the back of an unusually slick campaign (Pauline Hanson’s flakiness and the candidate Matthew Stephen’s travails notwithstanding).

If the polls’ reading of One Nation support is more-or-less accurate (big if), and if their preferences flow as they did when the party directed them to the LNP in local seats in the November state election, Labor could effectively be starting as much as 2% behind the eight-ball.

In Braddon, however, extenuating circumstances are harder to identify.

Certainly the Liberals underperformed in 2016 among Braddon voters, who have the lowest level of educational attainment in the country, and responded poorly to a Liberal campaign built around a presidential Malcolm Turnbull and the theme of “innovation”.

Even so, the seat has been excruciatingly tight in recent times, changing hands at four of the last five elections, and should thus be a relatively easy mark for an opposition party in the context of a byelection.

If Labor can’t hold on, there will be no avoiding the conclusion that regional battlers, who hold the key to a string of decisive seats along the eastern seaboard, are not responding to what Labor under Bill Shorten has to offer.