The weekend’s protesters were focused on the fourth anniversary of the invasion of Iraq, but for followers of Australian politics there was an equally interesting anniversary on Saturday: six years since the Ryan by-election.

Interest is more than just historical. Even as the implosion of the Queensland Liberal Party continues, the printing allowance scandal threatens the government with one or more impending by-elections in suburban Brisbane.

The interpretation of the 2001 Ryan result is crucial to understanding how the players will approach that possibility, and what might happen if it does come to pass.

Ryan, vacated by former defence minister John Moore, had a margin of 9.5%. It fell to the ALP – just – with a swing of 9.7%.

The most common interpretation at the time, still repeated today, is that this was an unmitigated disaster for the government – indeed, that it was the high point of Labor’s fortunes in that tumultuous year.

Wiser heads, however, pointed out that in comparison with other governments on the ropes in similar circumstances, the Howard government had done remarkably well.

The Canberra by-election of 1995 recorded a swing of 16.1% against the Keating government. Twenty years earlier, Bass had swung 14.6% against the Whitlam government.

So by comparison, keeping the swing under 10% was some achievement. It was the first sign that Kim Beazley’s ALP was not all-conquering, and that, given a sufficient stroke of luck for the Coalition, it could perhaps be beaten.

Five months later, the stroke of luck arrived with the Tampa, and the rest is history.

So there are hopes and fears on both sides about any by-election for Andrew Laming’s seat of Bowman, where the margin is a very Ryan-like 9.1%. (A by-election would be fought on the old boundaries; the post-redistribution margin is 8.9%.)

For Kevin Rudd, it offers the opportunity to score a killer blow like Bass or Canberra. But it poses the risk that failure to meet what are now alarmingly inflated expectations could bring on the same sort of downward spiral that consumed Beazley.

For John Howard, who must consider trying to force a by-election, it would be a correspondingly huge gamble. Can he repeat 2001? Or would he be better not trying to push his luck too far?