From today's DU front page article:
Credibility, as many have quipped, is like virginity: once lost, it is impossible to regain.http://www.democraticunderground.com/articles/04/03/25_well.htmlNo matter how many times that cliche is used, it's wrong. Credibility is subjective, and has many shades of grey. Some sources are more credible than others, but the positions can go up and down (Lou Dobbs seems to be gaining credibility to many here, for instance).
To take a current example, few here think Richard Clarke is discredited, although there's the statements he made in 2002 about how seriously Bush's regime had always taken terrorism. We weigh up the evidence, and make a judgement that his statements now, under oath, are more accurate.
This means that Clarke's testimony is far from a killer blow. Since it comes down to judgement (how much a threat terrorism was), it's just a bit more doubt on the suitability of Bush and his gang for office. The accusation that Iraq is a diversion from fighting al Qaeda, or terrorism generally, was already out there; again, this just adds a little more weight to it.