On 20 September 2012 the Senate voted against marriage equality by 41 votes to 26.
ALP voters overwhelmingly support marriage equality.
Eleven ALP senators, about one-third of the party's representation in that place, voted against marriage equality. Enough to change the outcome of the vote.
Nine of those 11 are former union officials.
Four are former officials of the Shop, Distributive and Allied industries union (SDA) - Bishop, Collins, Farrell and Hogg.
No doubt these Senators all voted according to their deeply held beliefs.
But you can't help notice the pattern.
There are about 1.2 million retail employees in Australia, about one in five are members of the SDA; are retail workers, unlike the rest of the voting population, more likely to oppose marriage equality? Ditto transport workers.
A small group of powerful union officials have considerable influence over pre-selections particularly for the Senate.
Marriage equality would be much more likely to pass the Senate if the ALP had a more open pre-selection process.