With Dean, you could show long sections of the video, but as long as you only played the microphone he was speaking into, you didn't hear the significant crowd noise to which he was responding. With Romney's quote, if you play longer versions, the impact of the expression is muted significantly. The real reason the quote has power is because it is consistent with impressions people ALREADY have of Romney. The "macaca moment" changed a perception, and quite quickly.

The real reason the quote has power is because it is consistent with impressions people ALREADY have of Romney.

Karl Rove popularized the idea that the best way to bring down an opponent is to attack a strength - done with devastating effectiveness with the Swift Boating of John Kerry. Romney's main claim to fame is his understanding of the economy based on his stint as governor and his experience as a business man. So this particular line of attack is going to have staying power. Romney made a mistake in trying to refute it as an "out of context quote" even though there might be some truth to that (personally I thought it was a thin reed to stand on though). It would have been far better if he had just acknowledged that it was a stupid thing to say - of course he doesn't like to fire people etc, my wife says I say the dumbest things sometimes etc..
A sincere mea culpa would not have kept people from talking about it or attacking him for it but a little humility and acknowledgment of error would have limited the damage and made the attacks less effective.

Believe I said that.

They first create impressions, true or not, then they comb the record looking for "evidence" to support the impression they already created. More than one right winger, when exposed with the falsehood of an accusation, has responded with something to the effect of "well, is sounds true" or "it should be true".