The dominant Victorian arm of the National Union of Workers has formally resolved to back progressive ALP delegates in their conference fight with Julia Gillard over gay marriage, drawing the Left to within striking distance of slapping down the prime minister’s preference for a self-defeating conscience vote.

Crikey can reveal that at a high-level meeting of NUW delegates held last night at the union’s Docklands offices, the seven-strong group formally resolved to vote to remove references to John Howard’s Marriage Act from the party’s platform and object to the PM’s conscience cop-out which would effectively keep gay nuptials outlawed in league with Tony Abbott.

To prevent any change to the status-quo and placate the religious Right, the PM is gunning for a re-write of party rules to allow a conscience vote while retaining the draft platform’s current discriminatory references. In the interests of solidarity, Labor currently only allows free votes in the case of abortion (broadly interpreted as pertaining to “issues of life”).

But last night’s decision binds the NUW’s seven delegates to the position of the National Left’s 177 disciples who are broadly committed to rejecting both the conscience proposal and the retention of the clause that states same-sex policy must be formed with reference to the Act.

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There is now a distinct possibly a further three or four NUW-aligned delegates from other states could fall in behind their southern comrades. And a further three notionally Right-aligned delegates — Yan Yean MP Danielle Green, ACT Deputy Chief Minister Andrew Barr and respected Victorian plumbers union scion Nathan Murphy — are all expected to defy Gillard on both fronts.

Senior sources say further defections are still expected from the Australian Workers Union, whose leader Paul Howes has advocated a free vote on the issue, and the Transport Workers Union, as reported last week. Right forces are believed to have abandoned hope of avoiding the excisions to the platform and are now feverishly texting delegates to come in behind the rule change.

Despite this, Crikey understands the PM has this morning been personally calling Right-aligned delegates in a last-ditch bid to head off moves to alter the platform.

The Left needs to scrape just 20 votes from the Right to command the numbers on conference floor. A majority of 201 of 400 delegates (assuming no-one is outside scoffing sausage rolls) is required to pass motions. On gay marriage there is expected to be some minor push back from five or six senior Left delegates, such as Senate leader Chris Evans who might be concerned about the optics of rolling the leader.

Procedurally, that could lead to serious problems. Saturday’s agenda shows that the platform debate is reserved for the morning session on Chapter 9 of the draft platform entitled “a fair go for all Australians”. But the related rules discussion is not scheduled to be held until the afternoon, raising the prospect of delegates voting for a slapdown of the PM in the morning and reversing their sentiments after lunch.

However, a senior NSW ALP Right source told Crikey this morning he hoped the “group of adults in the room” would alter the agenda and debate gay marriage in one go, presumably in the morning, to avoid any perceived inconsistencies.

A senior Left Victorian source hit straight back, branding the suggestion infantile: “That doesn’t sound like a very grown-up way of making the policy of a major progressive party on an issue that’s critical to members and supporters.”

They also noted two votes would still be required because the socially conservative Shop Assistants’ Union wouldn’t cop a resolution on platform changes combined with a conscience vote.

On other talismanic progressive issues, such as stopping Gillard’s proposed lifting of the party’s ban on uranium exports to India and the direct election of delegates and office bearers, the mathematics are more problematic — deviations from both the Left and Right would likely cancel each other out.

The official delegate list, obtained by Crikey, makes for interesting reading and is expected to be daubed with frenzied highlighting and crossing out in the lead-up to Friday’s kick off.