Shortly before he became Prime Minister in 2008, John Key claimed he was more liberal than he looked. Or - to be more exact - more liberal than his voting record on conscience issues suggested.

At that stage of his political career, Key had voted against separate pieces of legislation authorising civil unions and the decriminalising of prostitution. He had supported a failed bid to make euthanasia permissible.

In deciding his stance on conscience votes, Key made no apology for reflecting what he considered to be the majority opinion in his Helensville electorate.

As he acknowledged yesterday, however, as National's leader, he does not enjoy the luxury of being able to cast a conscience vote in isolation.


He has to consider his party's wider interests. That was first apparent in 2007 when, as Opposition leader, he brokered a compromise which enabled Sue Bradford's anti-smacking bill to become law - a move which secured kudos for him and National, but also no small amount of criticism.

Fast forward five years and Key's decision to swing wholeheartedly behind Louisa Wall's "marriage equality" bill likewise will have followed careful thought and analysis - not some rush of blood to the head.

The Prime Minister says he will vote for the Labour MP's private member's bill through all its parliamentary stages after initially indicating he would back it to the select committee stage while reserving his position on going any further.

Key's change of mind is in part due to him coming to the view that gay marriage does not undermine the concept of marriage.

Gay marriage does not impact on his or most people's daily lives. There is less political heat in the issue than was the case with Bradford's bill which had relevance for every household with younger children.

It is also a matter of consistency. You can hardly tout for votes at the Big Gay Out only to be ambivalent about same-sex marriage. Moreover, if gay marriage has Obama's backing, who is John Key to quibble with that.

Backing Wall's bill carries some risks for National. To become law, the bill is probably going to need the parliamentary votes of about 10 or so National MPs. They could find themselves heavily targeted by opponents of gay marriage - which is why they are reluctant to declare their position now. Key's stance will offer them some cover, however.

Key has clearly judged public tolerance for such measures is now higher than it was before National returned to power. Back then, Labour was a tiring, third-term Government. The anti-smacking legislation became a symbol of what was perceived as Labour's overbearing political correctness. Helen Clark's decision to take Bradford's bill on board as a Government measure only accentuated that perception.

In contrast, while Key will vote for Wall's bill, it is not a National or Government measure. He is under no obligation to ensure it becomes law.

Wall's bill is a cost-free means of portraying National as a "modern" party. Key may have another motive - giving Colin Craig's Conservative Party some room to National's right to carve out a niche.

Key's backing for Wall's bill also provides some balance to National's more overtly right-leaning policies such as welfare reform.

It is always possible that majority public opinion which, according to polls, currently supports a law change to allow same-sex marriage, could suddenly shift markedly the other way.

But Key is gambling - fairly safely - on that not happening. In doing so he is out to show social liberals that National is more liberal than they think.