What caucus giveth, caucus can also taketh away. Bill Shorten will not need reminding of this baleful detail as the countdown begins this week for what are, arguably, the most consequential byelections since Labor lost the seat of Bass in Tasmania in 1975, heralding the beginning of the end of the Whitlam government.
More than a generation later, another Labor leader is facing a similarly testing byelection moment – not from government, but from opposition. Byelections should benefit oppositions since they provide an opportunity for voters to register a protest without actually changing a government (the average byelection swing against the government since 1901 has been 3.8 per cent); but in this case pressure is not on the government of the day but on the opposition.
This is largely because of the peculiar circumstances under which these five byelections are being held after High Court rulings on the ineligibility of sitting members under Section 44 of the Constitution.
Four of the byelections are in seats won by Labor in 2016, and one by an independent. Labor seats are Fremantle, margin 7.5 per cent; Perth, 3.3 per cent; Braddon, 2.2 per cent; and Longman, 0.8 percent.
Labor will win Fremantle and Perth, but results in the Tasmanian seat of Braddon and Longman on the outskirts of Brisbane are unpredictable, and thus politically precarious for Shorten.
The other seat is Mayo where Rebekha Sharkie of the Centre Alliance appears on course to retain her seat against Liberal candidate Georgina Downer, whose pre-selection in the Adelaide Hills seat held by her father, Alexander, looks questionable, to say the least.
In his refusal to acknowledge questions about the validity of sitting Labor members and thus resolving their status a long time ago, Shorten has only himself to blame for what is, potentially, a toxic moment for him.
These byelections could have been got out of the way when eligibility issues arose rather than allowing the government to group them together in a way that is perilous for Labor.
The timing for Shorten could hardly be worse, with a general election pending within the next nine months.
If Labor loses both Longman and Braddon, he will inevitably face questions about his leadership, and, more to the point, whether Labor can regain government under his dead weight.
The latest Ipsos/Fairfax poll is a poor result for Shorten. Whatever might be said about usefulness of the preferred prime minister measure, a widening gap, now of 27 points, between the Opposition Leader and Malcolm Turnbull - up from 18 points last month - cannot easily be dismissed.
A persistent polling message seems to be this: the more people see of Shorten, the less convinced they are about him.
The poll also puts Labor and Coalition now neck-and-neck on two-party preferred at 50-50 - up four points for the Coalition on last month. Labor’s primary vote is down a point to 34 per cent. This is not good news.
In the midnight hours for Shorten's caucus colleagues, not least of concerns will be that Labor's narrow defeat in 2016 would mark the zenith of his ambitions from which he would slip back.
When he was elected Labor leader in a caucus and party-wide ballot in 2013 after Kevin Rudd lost to Tony Abbott, expectations were he would serve one term, lose the subsequent election and be replaced by a more electorally appealing alternative.
His outperformance in 2016, thanks partly to a dreadful campaign by Turnbull, whose political amalgam of haughtiness and confected affability held little appeal for voters, prolonged his tenure.
Now, a moment of truth has arrived.
My sampling of caucus opinion suggests there is little or no appetite for change given the transactional costs internally of such a shift, not to mention the opportunity this sort of disruption would provide for Turnbull to call an early election.
Nor is there is there as much enthusiasm within Caucus for Anthony Albanese as an alternative, as the media would have you believe. Another potential candidate, Treasury spokesman Chris Bowen, is not without his detractors. Bowen's role as Kevin Rudd's numbers man in the last Labor leadership eruption has not been forgotten.
However, political sentiment can shift quickly, as we've seen. Rudd's "poison pill" caucus rule change that requires 60 per cent of its members to demand a leadership ballot is not written in stone – nor in black letters in Labor's constitution.
The Rudd rule could be abandoned with a simple 51 per cent caucus "taketh away" vote.
Adding to complications for Shorten in this critical week for him politically is a civil war in his own Victorian branch over pre-selections and skirmishing by factional warlords.
This is hardly the best advertisement for Labor unity – or Shorten's control over his own warring Victorian division, whose industrial and political wings are at each other's throats.
Having said all that, history remains firmly on the opposition leader's side. The last time a government won a seat from the opposition in a byelection was in 1920, 98 years ago, when the Labor member for Kalgoorlie had been expelled from the parliament.