If you want to understand where the Romney campaign and their backers see the Medicare fight, read this Wall Street Journal editorial, whose closing paragraph summarizes their mindset nicely:
That’s the real reason liberals and the press corps claim to be so upset by the Romney Medicare ad. By governing so far to the left, Mr. Obama may have neutralized Mediscare and made voters more receptive to center-right solutions. Medicare is already changing because it must. The difference this year is that Republicans have a plan to save it.
In short, Romney-Ryan believe they can play the trump card of ObamaCare to neutralize the Democrats’ attacks.
As I wrote last night, Republicans aren’t sure this will work — but they are now committed to trying.