The ad hominem is accusing the poster of supporting the reelection of Obama because they noted a reservation about Romney. Noting that a non-vote for Romney has that effect is one thing, but accusing of a pro-Obama bias (on FR, at least) is insulting to anyone who is struggling with the distaste they have for the republican candidate.
Phrasing the accusation as a question is merely a rhetorical device.
The reason why Romney is less believable on his change of mind is the history of such position pivots, depending on which audience he has been facing at the time.
Yours is not an egregious example, but it is a very common one. I am a reluctant supporter for Romney - and much more enthusiastic for Ryan. I have been fielding such accusations for months - and I suppose I’m touchy about it.
Romney or Obama will be the next President. Spending all their time trash talking Romney with absolutely fraudulent talking points does one thing help Obama.
It might not be the intent of the posters doing it, it is the result of their actions.