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It is the first big unexpected moment of the US presidential election campaign.

I was inclined to scoff when ABC called their special news report a "historic political and cultural moment", but on reflection the hype is right.

President Obama, who had looked a little lame suggesting that his position on gay marriage was evolving (and ) has now come out personally, firmly, in favour.

He said at one time he thought that civil partnership was enough.

But now: "I've just concluded that for me personally it is important for me to go ahead and affirm that I think same-sex couples should be able to get married."

He said there was a generational change and cited his daughters as an example.

"There have been times where Michelle and I have been sitting around the dinner table and we're talking about their friends and their parents and Malia and Sasha, it wouldn't dawn on them that somehow their friends' parents would be treated differently," he said.

"It doesn't make sense to them and frankly, that's the kind of thing that prompts a change in perspective."

There is a risk in this. For some it will further brand Mr Obama as the devil in disguise, destroying the fabric of America. That sharpens the election, but it does no harm to his campaign.

Most social conservatives will not vote for Mr Obama anyway. But there are natural supporters, like African-American evangelical Christians, who won't like this.

North Carolina, an important swing state, has just voted to outlaw gay marriage and partnerships.

But Obama explicitly mentioned his religious faith:

"When we think about our faith, the thing at root that we think about is, not only Christ sacrificing himself on our behalf, but it's also the Golden Rule, you know, treat others the way you would want to be treated."

While not doubting that the president has struggled long and hard with this, this seems to me a deliberate moment.

Maybe the timing was rushed, but I am sure the campaign had looked at this again and again. I suspect they calculated this will fire up liberals, and make them more likely to vote.

It is after all not only a very divisive issue, it is something for the future, something that can "evolve" still further in a second term.

The US is a when it comes to gay partnership rights.

At a federal level, the one thing that could be done is to repeal the Defense of Marriage Act. The law means that other states don't have to recognise rights granted by one state. There is no sign, and perhaps no likelihood that Mr Obama would back repeal.

But it is the logical next move and I would expect campaigners will want to know what his personal views mean in term of government policy. It's not just about the economy, after all.