It’s quite pitiful, and also simply not credible. If they were really as concerned about getting conservative policies enacted, there are any number of constructive things they could do in House and Senate races to make a difference and just ignore Romney altogether. Instead, they just do what they always do - act miserable and try to make other people miserable. On another thread showing a tight Senate race in Indiana, they seemed almost excited about the prospect of Mourdock losing so they can could then blame it on Romney just so they could hopefully come along and rub a loss in people’s faces. They simply do not come across as sincere. Romney is just an excuse and a means to carry this out - not the reason.
My current best guess is that the Mitt-haters are motivated by their belief that Mormonism is demonic and that this trumps all other considerations.
They believe that the GOP, by nominating Mitt, has literally made a pact with Satan and must therefore be opposed to the utmost.