I'm not too keen on Romney, and yes, they would attack everything about his religion. The Democratic Party is absolutely immune to hypocrisy, and the press will see they aren't called on it.
Unless you are also a Mormon, to the average person Mormonism appears to contain some doctrine that sounds about on a par with Heavens Gate. The outlawing of polygamy may be long-established official doctrine but all sorts of 'Breakaway sects' have continued to practice it to the present day.
Romney certainly isn't going to appeal to Second Amendment types, the only thing he might do on the plus side is bring in a couple of traditionally-Blue New England states, which I regard as an extreme longshot. The Pro-Life crowd will vote for a GOP mailsack over the Dem Duo, so Romney isn't bringing anything to that for the GOP that they don't already have. If McCain carries any of the northeast, it would more likely be because of Bradley Effect rather than Mitt Love.