IMNASHO: Attacking *credibility* is legit. It's up against the line, but this side of it. One I think would be a bit *over* the line would be "You linked to Fox News, why should I believe anything YOU say?" Or more overtly, "Michael Moore supports that cause, therefore it must be a bad cause." (That's a double fallacy - also post hoc, ergo propter hoc, of a sort...)

The most blatant variety is "Fox News is always wrong, therefore when they say it's gonna be sunny today, it's gonna rain" (which ignores (a) that big ball in the sky and (b) the fact that bias != wrong). (That last is shocking, I know.)