Marriage equality is poised to move a step closer despite a final push by opponents.

The same-sex marriage bill will go before Parliament for a second reading tomorrow and, on current numbers, will probably pass. If it does, only a third and final reading stands in the way of it becoming law.

Opponents of the bill have been targeting potential swing MPs in the leadup to the vote, with Family First printing 150,000 pamphlets urging people to lobby 35 possible defecting MPs, 21 of whom are needed to defeat the bill.

Family First national director Bob McCoskrie yesterday accused Parliament of ramming through the legislation just as public opinion was turning against same-sex marriage.

"They want this vote through at speed so they can get it off the agenda," he said.

Family First has launched several late salvos against same-sex marriage, obtaining a legal opinion last week which described the bill as "unprincipled and wrong".

But at least four possible defectors on Family First's list have already committed to supporting the bill through its second reading.

Cabinet ministers Judith Collins and Maurice Williamson were both targeted because of their opposition to legalising same-sex civil unions in 2004.

Yesterday both confirmed their support for the bill.

Prime Minister John Key, viewed as another target, has also previously said he would support the bill. National MP Paul Hutchinson said he had faced "considerable pressure" from Family First to switch but had not been swayed.

- © Fairfax NZ News