Your reputation, credibility and timing are sometimes more important than your message.
Dan Hannan makes a very good point here about catching the wave of popular opinion and the importance of timing.
Once people categorise you as good, bad, credible, deceitful etc. there is a huge job awaiting you before they will even hear your message. And then there is still some mileage to do before they will consider believing you.
David Cameron understood that. Brown’s credibility has run out. How angry he must be when he considers that Blair used up almost all of Labour’s Credibility Credits before Brown became PM.