It's the Romney Sleaze Machine Prime Directive in action: don't address the message, attack--or in this case impugn--the messenger.
What’s there to refute? I could put out an ad that notes that Fred Thompson was the reason CFR passed, so when he now claims he thinks it was wrong, I just can’t trust him.
What could he say back? It’s true that he supported CFR, he spoke for it, he voted for it, he was the reason it passed the Senate. All he could say is that I was wrong not to trust him now. But what kind of response would that be?
Romney has already “responded”, because he has stated his position, and his position is a refutation of ad’s warning that Romney won’t be pro-life.
Romney will be as pro-life as Fred Thompson, probably a little more. But if they want to not trust him, that’s not something that can be “refuted”.