I am disappointed that some one as reasonable as you could support attacking a man over his faith... I bet you would've been outraged has McCain gone after Rev Wright
It's not that I personally support going after random religions, but rather I am addressing it from a strategic point of campaigning. If they were going to do such a thing, then it should have been done in primary season so as to avoid as much potential backlash as possible.
I do, however, think that if a church or religious group engages in overt and official political action, funneling their money and manpower into campaigns in order to affect the result (most notably, Proposition 8 in California), then that group could hypothetically be viewed as more of a political entity than a religious one. It is at that point - and particularly when you consider that the religious/moral elements are being woven into the political dialogue - that criticism of the values and actions of the Mormon Church can be legitimately discussed in political terms.
Even though the Mormons aren't on your side at the moment, they may not be so antithetical to progressives in the long run. After all their church does believe in a constant revelation such that their beliefs could change dramatically at some point in the future with very little ramifications. It's much easier for Mormonism to adapt than most Christian sects with a less fluid concept of morality.
Would that really affect their voting behaviors, though? I don't think a change in position on a social issue or two (assuming you were referring to that) would necessarily sway Mormons to the left politically in any significant way.
It depends on the issue. But what's your goal, to get Democrats elected or determine the course of future of the country? If it's the latter you would gladly take anyone in the opposite party who comes to agree with you on anything over someone who doesn't.
Certainly, the DNC would, but not to the extent that fundamental values of the party are radically changed in some way and it becomes barely distinguishable. No one in Obama's campaign would be foolish enough to suggest attacking Romney for his religion even though this demographic is far out of reach, but Utah hasn't gone Democratic since 1964. There is a very small fringe group of Mormons (i.e. Harry Reid) that vote that way. I think Mormons are fairly fiscally conservative (albeit it's a generalization) as well as socially conservative, so there isn't much for the Democrats to gain in this respect unless more Jim Mathesons get elected.