The Nick Xenophon Team on Monday joined the Greens in committing to vote down the plebiscite in the Senate, making Labor the government's only hope of honouring its election commitment.

Meanwhile, conservative and moderate wings of the Coalition have rallied behind Prime Minister Malcolm Turnbull for his rigid defence of the Abbott-era plebiscite, insisting it is "the only way" to resolve the issue.

Addressing Labor's new shadow ministry, Opposition Leader Bill Shorten railed against the "vile, negative campaign" that a plebiscite would unleash, and reiterated that Labor would continue to push for a free vote in the Parliament.

MPs did not discuss the issue at a Labor caucus meeting on Monday, but sources from the Left and Right factions indicated there was very little, if any, resistance to blocking the plebiscite.

One shadow minister suggested Labor could yet be persuaded to wave the plebiscite through if it approved of the wording - for example, if the question referred to "marriage equality".

It is also understood that consideration is being given to the view that Labor will be judged harshly for delaying gay marriage indefinitely and denying the public a chance to have a say.

Senate powerbroker Nick Xenophon delivered a potentially fatal blow to the plebiscite on Monday, confirming his party would vote to block the bill. "We believe this is an issue that ought to be determined by the Parliament," he said, adding that all four NXT parliamentarians support marriage equality.

While the government has the support of One Nation's four senators plus David Leyonhjelm, Jacqui Lambie and Bob Day, that will not be sufficient to get the plebiscite through the Senate if Labor ultimately votes to block it.

Marriage equality campaigners have called for a "reset" in the debate, pinning their hopes on three options: Mr Turnbull changing tack and granting his party a free vote, Liberal MPs crossing the floor, or some sort of multi-partisan settlement brokered between the parties.

All signs coming from the government indicate that none of those will happen. Education Minister Simon Birmingham, a prominent supporter of same-sex marriage, warned a plebiscite was "the only way" to achieve reform in the next three years.

The only person to address the issue of same sex marriage in a Coalition party room meeting on Monday was conservative Liberal MP Andrew Hastie, who wanted to "publicly applaud" Mr Turnbull's strident defence of the plebiscite.

"As Labor take the low road, we as the Coalition must – to quote the Steve Winwood 80s classic – bring a higher love," he told his colleagues.

But even ardent same-sex marriage supporters within the government such as Warren Entsch showed no signs of budging.

"We made a commitment on the plebiscite and we are sticking to it," he told Fairfax Media after the meeting. "If we don't do this, the debate will be about a broken promise rather than about same-sex marriage and this will be the 19th time this [same-sex marriage] goes down the gurgler."

Transport and Infrastructure Minister Darren Chester, another supporter of change, told Sky News he had promised his electorate a public vote and the plebiscite was now "the only pathway" to same-sex marriage.

On the Labor side, prominent same-sex marriage advocate Louise Pratt said the plebiscite was a tool used by those opposed to change, and should not proceed.

"I don't want young lesbian and gay people to have to go door to door asking for their civil rights," the WA senator told Fairfax Media. "It's the job of the Parliament to deal with this question."

- with Tom McIlroy and Heath Aston