IMHO, the Catholic church's leadership has lost much of the credibility it had. Continuing coverups of gay sex and child molestation by priests has left them appearing as morally bankrupt and unwilling to follow even their own precepts.
How much "credibility" does a person need, in your world, to say that it's wrong to suck a baby's brains out of her skull when she's three inches from birth?
I mean, John Kerry won't say that. He says that people who do that should be "proud" of what they do. But do Catholic bishops, or anyone else for that matter, really need much "credibility" to say that that's wrong, and someone who does it or supports it shouldn't present themselves at the altar as though they were a faithful Christian?
Basically, what you're espousing is two logical fallacies in one. The first is the "fallacy of composition" -- just because some bishops are bad, it does not follow that all bishops are bad. The second is the "ad hominem of detraction" -- "Joe Smith is a bad person, therefore I can disregard what he says". That's self-evidently bogus -- if a known murderer tells you that stealing is wrong, does that make stealing right? Of course not! For that matter, if a thief tells you stealing is wrong, that still doesn't make stealing right. Neither does it make killing kids right if a sexual abuser of kids tells you killing kids is wrong.
However, this is a pet trick of the left. It's the "tu quoque" ("you also") response they give to any attempt to take a moral stand against their side. They used it when they tried to deflect attention from Slick's lies about Lewinsky by pointing to Newt Gingrich's dumping his first wife, for example.
Any Catholic bishop who makes moral stand on his own authority, or "credibility," if you prefer, is making a mistake. Which Catholic bishop is without sin? Which Catholic layman is without sin, for that matter? Which Protestant layman? Nobody is! But if a Catholic bishop is teaching the doctrine of the Church, he's teaching something that came from a source Who is utterly without sin, and that Source said, "Thou shalt not commit murder." Since nobody can prove that abortion is not murder, that should be sufficient for any Christian to know that abortion is wrong, no "credibility" required.