Instead of making the conversation about issues where Huckabee is vulnerable and Romney isn't, the Romney campaign has guaranteed that for the next two weeks at least and probably beyond, the media conversation will be about, well, Mormonism. If there were more time before the actual voting begins, that might not be the worst thing in the world; they could get the wave of coverage out of the way, inoculate themselves to some extent, and then shift gears and start hammering Huckabee on taxes and immigration and so forth. But there isn't time: Christmas is coming, there's a very narrow window in which to define Mike Huckabee as a Mexican-loving crypto-liberal, and the Romney campaign has just ensured that everyone will be talking about the Urim and the Thummim instead of the Arkansas gas tax. Unless Romney gives the best speech in the history of speeches, I just don't see how that helps him win - in Iowa, New Hampshire, or anywhere.
I think it all depends on the speech itself. Romney will be given a national platform. How he handles it will be telling.