Nobody is waiting until Thursday's speech to weigh in on whether or not Romney is making a smart move. Marc Ambinder has a nice list of pros and cons, but I think Ross Douthat hits it on the head.
With the Iowa caucus on January 3rd, the primary campaign basically lasts from today until Christmas Eve. That's all the time Romney has to reframe Mike Huckabee, his top competitor in Iowa, who, due to his late rise and favorable media coverage, has been able to keep his negatives off the radar. Huckabee has three "problems" that could make him vulnerable in the GOP race: a relatively compassionate history with illegal immigrants, a decidedly moderate fiscal record, and a complete lack of foreign policy chops. Romney has the money and organization in Iowa to put these things front and center.
Instead, though, a significant portion of the next three weeks will be devoted to questions of faith. And when Republican primary voters are asked to make a decision based on faith, and their options are the socially conservative former Baptist preacher who speaks eloquently and authoritatively about the Bible or a Mormon guy who doesn't even have the principles to avoid waffling on small parts of his faith in order to make it more palatable to voters, who do you think they are going to choose?
And then there's the danger that this speech brings all of Mormonism's quirks to the fore. Like the fact that it didn't allow black people to become priests until 1978. Or the fact that it technically sees all conventional Christian churches as "apostates." Or the fact that it still teaches that believers can have multiple wives in heaven. Maybe not odder than the oddities of any other faith (except the racism thing, which originates in some pretty nasty anti-black scriptures), but definitely not the stuff Romney wants in his news coverage.
If this speech had come six months ago, voters would have had time to chew it over, digest it, and then move on to something else. But now Iowans will have all this bouncing around their heads as they go to the caucuses.